Thunder Warriors

The proto-Astartes, better known as the Thunder Warriors were the first gene-enhanced warriors created by the Emperor as the precursors to the modern Space Marines. Wrought to be living weapons, the Thunder Warriors were  known to be even stronger, more powerful and mightier than the later Space Marines themselves, except in longevity.

History
During the turbulent era known as Old Night, the Sol System and the nearby star systems that had been colonised by humanity during the Dark Age of Technology were effectively cut off from interstellar travel or communication with each other due to the massive Warp Storms that swept the galaxy as the Immaterium was roiled by the millennia-long gestation of the Chaos God Slaanesh and the turbulence that marked the decay of the Eldar empire before the Fall. During the five millennia of anarchy, fear and violence, Old Earth's once unified planetary government had completely broken down and been divided into dozens of inter-warring nations. Continuous warfare raged across the surface of Terra for two and a half thousand years, beginning in the late 27th Millennium. Little remained of the once sophisticated civilisation of Old Earth's glorious past as techno-barbarian warlords and their gen-enhanced warrior-hordes continuously fought over the planet, which had become little more than a massive battleground for their apocalyptic wars of attrition.

It was during this dark time that the Emperor of Mankind developed first proto-Astartes, creating them from the genetic material of the Primarchs. Twenty Legions of Space Marines were created, each from the genetic material of a specific Primarch. The Emperor wrought them to be faster, stronger and more powerful than any of the feral barbarian gene-sept warriors that claimed fealty over Humanity’s birth rock; soldiers to drag their world back from the anarchy into which it had fallen. It was against this backdrop that the mysterious saviour known only as the Emperor revealed himself -- the eagle and lightning banner marching proudly before His unstoppable armies of Thunder Warriors.

Unification Wars
Emerging from obscurity, the Emperor take a direct hand in the future of humanity, conquering the various warring factions of Mankind's homeworld and establishing his direct rule over the Earth. The Emperor accepted the deaths of the many innocents that resulted from his conquest with great remorse in order to achieve the greater good of unifying humanity and protecting it from the manifest predations of the Warp. Battles would last weeks on end, with body counts in the millions and duels of titanic warlords that sundered mountains and split continents. Future Imperial scholars would later dismiss these victories as lurid hyperbole, refusing to believe that such clashes of arms could possibly have been fought, but indeed they were. In the end, the Thunder Warriors faced an ignominous fate. Officially, the historical chronicles tell of the last battle of Unity, known as the Battle of Mount Ararat which was fought in the Kingdom of Urartu, that the Thunder Warriors were slain to a man. The famed Thunder Warrior, Arik Taranis the 'Lightning Bearer', raising the Banner of Lightning at the declaration of Unity before dying of his wounds. It was a measure of their heroic sacrifice that they had all died to win the last and greatest victory for the Emperor. Unfortunately, the events surrounding the closing days of the Wars of Unity are a blatant falsehood for history is written by the victors.

The Thunder Warriors had outlived their usefulness. They were merely tools; a means to an end. The Emperor knew it would not do for him to have to share his victory with others, for he needed the people of Terra to unite behind a single, dominant leader in order to begin his future conquest of the galaxy. There would be no glory or grand victory, conquering a world with an unstoppable army at your back. To begin his legend, and unite all of mankind behind his rule, the history books had to reflect that he won the war single-handedly. Unfortunately, there could be no one left alive to contradict His version of events.

There are many diverse factors to explain the dissolution of the Thunder Warriors. The Emperor may have deemed these early proto-Astartes a success; the conquering of the various nations achieved, the Emperor may have felt that the terror of these brutal warriors had simply outlived their usefulness.

The Emperor knew that he needed to be the sole unifier of Terra in order to build his Galaxy of Man. It would look bad if people realized the Emperor had an indestructible force behind him, and that it was not solely the Emperor responsible for unifying Terra. The Thunder Warriors understood only conquest but some of them had a profound understanding that they and the others like them were merely tools for the job at the time. They were superior warriors but not built to last and they had a flaw in the Emperor's grand plan.

The forced evolution that is spouted about would be difficult to achieve if you have a killing machine that perfect, let alone an army of them to conquer your foes. How would humanity get any tougher if the galaxy was at peace? Look where that led the Eldar. Another incident like that would likely destroy all of the material realm.

The Emperor no longer needed the Thunder Warriors anymore because the newly created Space Marines were created to be imperfect. They were susceptible to Psykers, Thunder Warriors were not. Space Marines were vulnerable to human emotion; Thunder Warriors were not. In order to sustain a fine balance of death, destruction, mayhem, malaise and the grim darkness of the current universe, and ensure long-term species survival, the Emperor needed to create a playing field level enough for Humans to live on and perhaps win. Otherwise humans would fall to the destruction or martyrdom or fall into a state of complete apathy will total victory, which would also result in a victory for the Ruinous Powers. Imperfect warriors enable the species to survive; Thunder Warriors would not. They are too good at what they do. Plus, they are a liability. Terra needed bloody handed armies to conquer bloody handed tyrants and psychic scourges, but the galaxy needed a level headed force able to conquer the worst opponents but also be able to stay their hands.

The explanation of the Emperor discarding the Thunder Warriors in favour of the Astartes created for the Great Crusade, lies in the design being perfected to provide a longer life span; provide easier mass production for the scale needed for galactic conquest (there have been similar references to why Astartes were preferred over Custodes for the legions), and to include the advances the Emperor salvaged from the Primarch project - which probably improved the design and served to create a greater bond of loyalty between the Astartes and their Primarchs.