Well, I think I can help with this one. I have not joined a league to date. I have limited online experience and when I did play, it was too choppy for me to confidently test my skills.
A second reason for not playing in a league boils down to a fear of commitment. My schedule is not condusive to participating reliably enough to commit to a season. And, as I've seen of late, people who join up and do not finish draw the vengence of all involved.
So I, personally, do not join simply because of common courtesy. I don't want to be the problem child or the "flaky" one. I come around here simply to read up on discussions, play new roms and add my two cents from time to time. It is a place that I come for enjoyment. It works for me. And, as far as I know, many others do the same. It has to be pretty difficult to fill the many leagues with reliable coaches so I understand where this thread is coming from.
Hopefully that helped.