a) There are TRILLIONS of us. b) We are EVERYWHERE.
Frankly we are the cockroaches of the 40k-verse (yeah, I know you thought it was the Tyranids but really it's us). No matter how many you kill there are always more coming out of the baseboards.
The Eldar are a dying race with no power center to coordinate action.
The Tau hold a tidy little corner of a very large Galaxy - and probably have no idea just how lbig the Imperium is.
Necrons are literally a dead race. A major threat but we have been able to take them a time or two.
Tyranids - for Emperor's sake shut off the LIGHT!