Doctrine, terrain, resource availability, distance to front line, factory level.
SPAA requires a level 3 ordnance plant to produce at optimum speed. That's expensive, and takes a while to build. Do you need AA at the front line right away? Your options are, upgrade the factories in your core, and drive the SPAA to the front line, maybe driving across Asia. Or build at the front line, at 1/4 speed, maybe getting only 1-2 units per day. Or spam 12 AA in 24 hours in a single level 1 factory.
Comparing power and build speed per level, a level 3 AA builds at the same speed as a level 1 SPAA, but the level 3 AA has a higher AA defense and a lower manpower cost. Goes back to the basic CoW truth than higher level "starter" units are better than level 1 "advanced" units. The same is true for infantry versus mech infantry, arty versus SP arty, etc. Higher level units give you more useful output per manpower invested. That's why generally speaking, "starter" units win, because they can be upgraded to high levels. The effect compounds when you don't waste your time and resources researching SPAA and other "advanced" units. Invest in AA research, get those units to level 3 or 4, and enjoy those excellent power-per-manpower ratios.
