Perhaps it's a population cap issue? Maybe folks are already capped out?
I actually got a citizenship petition after my fort hit 200 population. I got a few, actually. My fort got up to 203 people in total. I believe there were 2 elven poets (from a traveling troupe of performers), and about 7 humans. All became CITIZENS. They would stop performing in the tavern, and immediately run off to pick up some wood outside, or start fishing (whatever their default labors were). However, you still can't assign them as performers, scholars, militia captains, burrows, or noble positions. I didn't get one to experience a strange mood, but in a 203 strong fort I wasn't going to get any one specific guy to mood anyway. Most of them got drafted into the military. I didn't need more haulers and couldn't use them as bards outside the horribly saturated main tavern.
I can't remember if long-term residency petitions stopped at this time, but births did. However, I would still occasionally get citizenship offers over 200. I'm not sure if this is intended behavior or not, but probably is a bug. We don't get births and migrants over 200 outside of the monarch's retinue afterall. Oh, and I definitely didn't have a monarch. This was a 42.04 game with the exported wealth bug.
As for the fort, it was about 6 years old and I think the first citizenship petition happened around the third or fourth year. It was definitely a long-term thing. I actually didn't get much in the way of visitors. I never had much more than 10 visitors at a time in my tavern, and I might have gotten 2-3 scholars totals in those 5-6 years. The dining hall was engraved and somewhat large (10x20?), and I had begun engraving the rest of my fort too. I noticed that different people would have different reasons for showing up. Like Human McScholar was "curious about my library." Or this one human dancer (who later inherited a noble title in my tavern, but wouldn't leave for YEARS) heard "this was the place to entertain people." Some of them showed up to worship at various temples- I had made an (engraved) temple to every deity on the list, so about 23 in total. I churned out instruments (had one made only of bone, and another of bone, wood, and silk that I could mass-produce without fuel/metal), but no one played with them in the temples that I saw ; /