Some jams are good...
If they are, word gets around, and they get crowded...
Then, it's hard on the guy(s) and/or gal(s) running it because everyone wants to play and you'd be amazed how childish all these grown up folk can be when they feel like they aren't getting their turn...
Then, the host quits because he/she is tired of babysitting...
Process repeats at new jam where someone who used to complain to the old host just *knows* he could do a better job.
That's just my experience in the last 25 years of so of open jams. It doesn't always work that way, but it does often enough to see a pattern.
Fortunately we do have some great hosts and some great jams in Orange County, but because of that, they can get really crowded.