Well, they might also stop taking part due to comments like this one, to be honest. Because if what you took away from it was "people are not as fast as others, so they stop taking part", then I don't think you understand the actual reasons for them "stopping taking part", you're just kind of saying they're (well, we're) quitters who give up for no real reason.
BTW, here's the rest of what I originally posted above. I edited it out, because I didn't want to sound like a whiner (and annoy everyone with yet another of my lengthy posts that probably didn't need to be posted in the first place), but maybe I shouldn't have, as it might (or might not...) help to explain things a bit better (because I really don't think it's that hard to understand, nor it is as simple as to just shrug it off with "well they are not as fast, so they don't join". So just append this to my previous comment:
But the simple matter of the fact is that whether I am slower than I was or the people I used to be able to have a race with have become faster, way too many of my Raceroom races lately consist of me watching the front pack disappear into the distance with each lap, and if there happen to be cars behind me, they are in turn either just a touch too slow for me, as you said (or simply make more mistakes), so I pull away from them as well, or they are just fast guys that got caught up in the back and quickly catch up and disappear into the distance as well (and if I'm lucky, I even manage to not crash myself as they pass by).
It can get a bit frustrating if your races are regularly like that, because there's always that tiny voice in your head that keeps telling you "come on, what are you even doing here, you could spend your time doing something better than this" (especially if you have ten other things you might rather be doing as I tend to). Because while I certainly enjoy the company and the TS banter (which are the major reason why i *do* join the races), the racing...not so much. And then all it may take is one stupid mistake and you just can't continue anymore, no matter how hard you try to convince yourself to go on (and I would still like to think that I do not give up *that* easily). Especially if there are factors involved outside of the race (as in physically around you). And that obviously happens to others as well, so a lot of time, I see people around me disappear from the race and the "no man's land" grow larger. (I'm not trying to guilt-trip anyone about giving up, BTW, I really want to stress that out - see above, I absolutely get it. And for the record, yesterday's race for me was more of the "factors outside of the race" kind of thing, I certainly could've had a race there if I kept at it.)
Meanwhile I am still quite slow in rF2, but I lately tend to have races that are way more fun there, because there seems to either be more people close to my pace, or there are generally more people, so getting stuck in said "no man's land" is nearly not as frequent for me there, and the Thursday races (that make me miss Raceroom events) also often feature really interesting and fun combos, thanks to various mods. And I don't mean to say I'm bored of Raceroom content, but when you get to choose between a race where just by looking at the list of people involved you know you will quite probably just spend the whole race lapping on your own and watching your gap grow larger with each lap (pretty much the case with yesterday's race and with many other races), and a race with a unique car and track combination you are likely to have fun with even if you might still be slow, the choice can be quite easy, really. Sundays are easier, because there's no alternative, so I usually sign up just to add one more car to the diminishing grid (and, again, obviously to have fun on TS, that goes without saying), even if I fully expect to mostly spend the race on my own.
(BTW please note that I'm absolutely not complaining about being slow, even though I usually feel like that's exactly what I'm doing and that everyone must be absolutely fed up with it, especially since I think I can still be reasonably fast or at least consistent at times, so I really don't have much to complain about. I'm absolutely content with being slow, as long as it is still fun.)