From what I've heard, it seems IEMs are easier to design than open back headphones. That's why once the general tuning (neutral + sub bass boost) was figured out, Chinese companies were able to jump onboard with low manufacturing costs and flood the market with excellent budget options.

Thieaudio, Moondrop, Harmonicdyne, Sivga have all made attempts at open backs, but none of them seem to be able to challenge the status quo of Hifiman and Sennheiser yet.