Marder 1A3 and BMP1

Why is not the Marder 1A3 below Marder 1A1? Would be perfect if we can use BMP1 instead of wasting time researching Marder 1A3, which I consider its the same as Marder 1A1. Marder 1A3 is just a waste of lions and time.

Other way around actually. 1A3 is the better Marder, while 1A1 is the trash one.

I think his statement is why is the Marder1A3 not foldered below the Marder 1A so BMP1 and Marder1A3 are swapped in the techtree.