Main reason? MP drain against OST would be insane. Grens are 4-man, 30mp per model squads, whereas USF squads are both bigger and less expensive; combined with rifles being better (and cheaper) than grens, giving them the ability to MP drain OST that hard would make USF insanely overpowered.
This was my thought as well when someone brought this up on reddit. Rifles already outclass Grens at all stages of the game (except for the brief period in most games where Grens have an LMG and USF has not yet gotten BARs or LMGs of their own). Grens would have absolutely no chance against Rifles if a Sniper was dropping them to 3 models before the Rifle / Gren fight even started.
I am, however, not necessarily opposed to them adding a 2 or 3 CP Sniper to Recon Commander or something like that. Obviously it would be the main attraction of that commander and you'd never combine it with LMGs or a Pershing or something.