Because it was nearly impossible for the okw to clear garrisons.
the issue isnt that the okw have the flame nade, its that VOLKS have said flame nade... y'know the cheap infantry that also get the best AT in the game and a vet requirement reducer for 90mu? there are also reduced suppression bulletins for volks....
the result is a cheap, relatively durable squad that can counter tanks and garrisons while being no shirk in infantry combat as well... its a unit that can do it all (which is why its blobbed, same as rifles)
if it had gone on sturms, there would have been a garrison counter that was available just as early but also require more than one unit to counter most enemies
and for no extra cost outside teching up no the less