Call-ins don't share a build queue with T0 units, which lets you deploy infantry faster.
This was the primary reason (for me). Remember back then OKW still had 340 starting manpower and Volks were 250mp, which would have meant OKW would've been able to build a squad of Volks and then deploy a squad of Panzerfusiliers (even if they'd start on cooldown) practically simultaneously. Something OKW with its already oppressive starting power absolutely did not need to have.
Ill tell you OP why. When PF got reworked into what they are now, with the grand offensive commander. The Pshrek upgrade specially, started a huge forum war because mostly allied players said they are going to be volkshreks 2.0, OPAF to keep it short.
To keep the beasts tamed the dev team proceed to nerf the squad in rather stupid ways, like decreasing its squad size, make it buildable and other stuff.
That is not the correct order of events. Reworked Panzerfusiliers were changed to be buildable with all the "stupid" changes right from the very first version of the balance mod. Before anyone else knew it was our intention to give them Panzerschrecks and well before all the complaints about Panzerschreck Panzerfusiliers presumably becoming Schreck Volks 2.0 (which, as we rightfully predicted, have proven to be unfounded by the way).
http://www.companyofheroes.com/blog/2019/03/19/new-commander-update-commander-rosters-and-test-mod