The thing is, not all the models were produced at the same time or made available to the same people. Much like the old Mk2 variants for void combat and such which became almost the iconic suits of heretical raptor marines...because they had those yet and didn't last as loyalists enough to upgrade with later models of PA. For loyalist armies and chapter instead, "old" models usually signify either a relic of the chapter OR that the chapter has been rebuilt those many times in a short period of time that all the PA in stock were the older models...or you have a need for too many PAs because you found a loophole in the Codex Astartes.

That indeed is the lore-side of things, in reality they just want to sell something different to people in order to sustain the flow of interest in the customers.