My hypothesis
Even small raptors had long tails that they used for balance. T.Rex had an enormous tail.
I would make sense that as the longer, heavier tail evolved, the center of gravity would move backward, eventually reducing the locomotive effectiveness of the forelegs, which would become smaller as the tail became larger. While lizards are not dinos, we can observe how some lizards have evolved to run on their back legs, which could be seen as an early stage of how the dinos evolved.
To test this model I'd have to review raptor evolution to see how the front leg / tail size ratio changed over time.
So, T. Rex had small forelimbs because he had a big tail. This served T.Rex because it could run better without big forelimbs.