Problem is a lot of Transformers designs outside of G1 were kinda shit.
With Bayformers, you got shitty bot modes with everything looking like a walking trash heap with a bugface.
With BW, you have an extremely small cast, with no duplicate fodder (outside of maybe Cheets/Tiger) like the Datsun's or the Seekers, so they can't be repeated endlessly in rainbow colors for free EZ cash.
With other generations, you often have nonsense altmodes people don't like, or the entire line is based around some shitty gimmick like "everyone is combiner pants", which would be a bitch to replicate especially when people would probably only buy Optimus and Optimuspants.
Japanese original stuff is largely passed over except for Overlord, and only really him because of IDW.
IDW is basically just G1 Redone, though altmodes and designs shift wildly because the comic ran for so long and the comic also had to adhere to whatever Hasbro said, so looks changed frequently for some characters.
G1's just the easiest all around to do. Simple designs with very few gimmicks (or rather, the gimmick is "it transforms!"). What they turn into are pretty notable or popular real world vehicles. They don't have to worry about shit like producing a bunch of different minicons or Optimuspants or whatever the fuck. It's designs pretty much everyone likes and is familiar with, whereas only a few people are going to go for things like firetruck Optimus, or munkee not trukk, especially if they do things like make those garish as fuck TM Optimus'. I'm skipping a bunch of Iron Factory releases lately because it's all TM nonsense, if they're going to do munkee, they better be doing the original gorilla and not the weird colored shit.