Since you can't accelerate matter above the speed of light.
Apparently there is more than Einstein, we are now believing that there is faster than the speed of light.
Calculations show that it requires infinite energy to reach the speed of light for matter. Because of that, accelerating something to a speed faster than light is impossible.
What we now believe is that we can move the space containing the matter instead of accelerating the matter itself. That's the idea behind warp drive and all that stuff, but I don't think any scientists believe it's possible for an object to be accelerated beyond c.