First, it would put it in line with most factions where rocket artillery is tier locked.
Most factions rocket artilery is not doctrine locked already.
Second, it would avoid USF just rushing to it while Ostheer and OKW are still using infantry and waiting for tanks.
USF rushing what?
You can have sherman well before you can call-in calli.
If anything, calli would arrive about 5 minutes FASTER and by the time the first comes out now, there would be 2 now.
Third, (and this one is the least important) it paralells with stock, being a sherman variant, though the m10 and the bulldozer might get away with it unless they get a unexpected buff to their original aim (AT and AI/AB respectively)
Puma, MHT, 250, StuG-E, M5, M3, suxton, priest.
Explain them then.
It doesn't parallels with anything, its completely different unit with completely different role then anything USF have in any of their tiers. EZ8 and 34/85 are the only units that match your 3rd point and that is why they are tech locked. And by the way, tech lock made them both arrive faster.