Different people in charge. Also Rhino is much older, much bigger, and has been developed by a fairly large team, whereas GH UI is just one person.
Feature and option creep are discussed for Rhino, and everyone seems to dislike it, but in the end taking a risk seems to be harder to justify when it’s the company’s money-maker and there’s 50 colleagues and 50 times more users who might get mad at you.
One reason I’m rewriting GH2 from scratch is not just that the code is becoming unmaintainable and unimprovable, but that it will give me a chance to get rid of 10 years of bad UI and feature decisions.