In NA at least most of the secondary leagues were populated by players who were looking at getting into the scene, had just recently moved into ESEA and were still playing in the other leagues because they always had, or had friends who did, and/or they were players who were taking a break or had fallen out of ESEA.
TWL, UGC, and CEVO post ESEA move were all part of that system. There were also an assortment of very small community based 6s leagues that were basically an avenue for players who consistently played pubs together to try out sixes with their friends. Most of these really small leagues only ran 1-2 seasons, as most of their player base either gave up or moved to larger leagues in search of more competition.
The issue for us here in NA is that the "new" player base is just much smaller than it used to be, and more people are aging out of the system altogether where they don't have time to even play a match a week in UGC etc.
As far as why specific leagues died:
The small pub community leagues: interested players moved on to larger leagues, and some stayed, but in either case most of the pub communities of that era have died because of quickplay options.
TWL died in part because it was, at one time, the go-to league to play in before going on to CEVO which was then the primary league, but when highlander really got going in the wake of the ETF2l HL tourney, UGC was the place to go for that and TWL, which had lost much of its player base to CEVO and ESEA by then, fully expired. Interesting fun fact Dr. Shadowpuppet, my fellow hoosier, was at one time the head admin for TWL.
UGC isn't fully dead yet but it has decayed fairly markedly in the past several years, in part due to the decline in popularity of highlander, its primary league, among other issues. I was the head 6s admin there for 2 seasons.
CEVO died multiple times and that is kind of a vexed story so I'll go over it in order.
The first death.
CEVO was the go-to league for the first really big run of comp 6s in NA (after early TWL and fragmov or whatever it was called). It had a lot of momentum having been a major league for CS 1.6 play for years. However, their tf2 league ran into several problems having to do with admin favoritism. Essentially certain rules were applied to certain other teams unevenly based on the fact that team x may've had an admin or an admin's friend playing on it, which soured many players on CEVO generally, and ESEA picked up tf2 at around the same time and promised an eventual tf2 LAN which more or less precipitated a mass exodus of players from CEVO to ESEA - furthermore, ESEA's admin system is automated, and several of their admins had no real interest in TF2, so players were less likely to encounter admin abuse.
CEVO continued on as a feeder league to ESEA with most of open playing in CEVO as well as ESEA, bolstered by plenty of teams that didn't want to pay ESEA's fees and "learn the ropes" as it were. However that all came to an end in 2011 IIRC, because they set up a playoff during Christmas vacation, and the "default day" for the first round of playoffs was Christmas Eve, so craploads of teams refused to play and couldn't work out a scheduled date to play due to the chaos that generally surrounds people's schedules around Christmas time in NA. That particular playoff ended with somewhere around only 5-6 matches being played in total out of a huge bracket of teams. The CEVO league was then formally declared dead.
CEVO then returned with a new angle, rather than run a league, they would run a "vanilla tournament" this was in part off of the back of the hype surrounding NA's first foray into European play that would take place at the I series LAN. Tons of teams signed up, a small entrance fee was to be paid, and nobody really had any problem with that. The tournament was also specifically set up so as to fall during ESEA's off-season, so teams wouldn't have to worry about scheduling conflicts. However, CEVO delayed the tournament on the grounds that they wanted more teams. They eventually delayed another week. Several teams began to openly discuss dropping from the tournament due to the likelihood that ESEA's first week would fall within the bounds of the tournament. CEVO then announced that the vanilla tournament as such would be cancelled, and that a "vanilla season" would instead be played. Almost every team dropped from the tournament and likewise requested that their entrance fees be refunded. In the end only 3 matches in total were played, and the "season" was won by yz50 and dummy's srslybro team they pocketed somewhere around like 300$ iirc lol.
Then CEVO was resurrected in the wake of ESEA's bit-coin bandit scandal, as well as, in part, some controversy surrounding some of ESEA's then recent "invite conduct" policy punishment schemes and penalties to winnings which occurred as a result - Pyyyour campaigned under the "no exceptions" catchphrase to voice his and others discontent with those series of decisions. CEVO was bankrolled by Nahani and several other people who had long backed TF2 and the league got off to a pretty hot start, being more or less on par with ESEA for coverage and team numbers. It also featured a LAN. However, the league encountered several problems, but the *most* important seems to have been CEVOs forfeit win policy which held that no team could be granted a forfeit win, and that any forfeited match would be replayed against another team which had also received a forfeit win. This meant that in some cases one could see 3-4 matches being played on multiple maps within the same week, as your team was forced to resolve last week's forfeit matches. Although there was widespread discontent relating to this rule's implications, nothing was really changed, and ultimately CEVO died a 4th time and has since remained defunct.