Its simple, you can’t use a grid that has the same key assigned twice, e.g. building a basic structure for Protoss: the Gateway is ‘g’, what if you assigned the Forge also to ‘g’ ? which key is going to be used?
But the main problem comes from trying to change a shared key like Stop ‘s’ to something like ‘t’.
For units like HT, Arbiter, Marine, Firebat, they’ve Storm, Stasis, Stim, all assigned to ‘t’ and ofc the new Stop command ‘t’, the game just won’t work this way.