I probably have a unique perspective. Was never a member back in the 2007 era. I strictly played f2p back then, got bored after the main progressed down to slow grinds with not much to do in f2p. Made some pure account builds for fun. Hated the removal of free trade and wildy. Though the BH cave was kind of fun on initial release, since some of my pure accounts were just right levels for lowest cave.
After FT removed, I was down to just merching on GE which was surprisingly profitable, especially early on. At one point I have like 12 accounts flipping items in f2p. Loved it when free trade can back so I could finally buy some good rares like phats and stuff. After getting several phats, felt sort of pointless flipping still, and flipping in f2p got much less profitable once everyone was doing it.
I did randomly check on updates once in a while even after I was basically quit. Saw the "oldschool" poll which got me excited since I would love starting fresh on new server. But it didn't get enough votes for f2p so I was a bit sad and let it go. Few months later I'm reading RS game updates and I see this new thing "bonds" coming out. I look at my giant stacks of RS3 GP just sitting there among those old accounts and see that members is good for both games. The rest is history.
So yeah back to the question. Main reason I play OSRS is because that's the game I actually learned how to play as a member for the first time, and my game knowledge grew as OSRS grew. I go to RS3 to buy bonds from time to time and I don't even recognize anything. The spam for dailies and the MTX I just find super annoying coming from OSRS, and the game interface is really intimidating. Maybe someday I will try actually playing a RS3 account on members, but probably not.
I guess I really like the player driven aspect of OSRS. Like how stuff has to pass polls, so devs really try to listen to players for the most part. And how you have a crowdsourced client in Runelite where useful new features come super fast without needing to wait for the Jagex team to get around to updating the client.