TLDR: Callins have become the meta, and have completely circumvented and made redundant the core tech/tier progression that is such a large core part of the game, and also which defines the factions. They are much more cost efficient AND stat superior to tier units, which has resulted in them REPLACING tiering, rather than adding variety to what is available through tiering. Finally, due to the above, non-callin Commanders have been marginalised, because though their abilities are an awesome source of diversity and options, they simply cannot compete with their normal tier units, against the cost efficiency and performance of callin Commanders.
I think there is another possible reason for people ignoring later tiers (although clearly they are able to do so because of late-game call-ins)
For Soviets it's this:
The Axis counters to loads of infantry / snipers = vehicles.
Soviets have a T2 counter to vehicles if they go T2 (AT gun) and a call-in counter to vehicles if they go T1 (guards)
This means that the hard counters to the STALL-TIL-ISU strategy become only soft counters. You just can't punish a 25-minute T2 strategy with tanks because they can field AT. If AT was at T3 (like amis in coh1), soviets would be forced to go T3. And if guards didn't have "button" (seriously, how could this affect a tank reversing? If you sit back and think about it...), then T1/T2 strats would be punishable with PIVs and pumas.
Unfortunately it's just not that punishable to spend 20 minutes with only T0-T1/T2 units.
TLDR
I think moving soviet AT to T3, increasing CP of mega-call-ins and removing button but increasing penetration of PTRS would massively change the meta, without tying call-ins to tiers.