trooper6:
I wonder if it has something to do with the character creation process lending itself to a bit less investment. I mean, you can easily just say: "Um...Catperson Swashbuckler, go!" And have no more characterization than that. And even if you have an involved backstory, it isn't going to really show up on the character sheet since (maybe in the quirks) since DF is very, very template based.
So maybe GURPS players are less invested in those PCs and more apt to abandon them than if they were custom made?
I don't think that's the case (at least, not necessarily). In my multiple failures at running play-by-post DF-style games (i.e. GURPS games that were based on d20-style adventure modules), I took the standard route to character creation of giving everybody X points to do with as they wished.
That didn't seem to evoke any greater level of commitment (for either the players or for myself) than using templates, unfortunately.
That said, I think that one reason GURPS arena games like Arena of Champions, Zuddha Yuddha, etc. have done well is that their format is much more friendly and compatible with the medium of play-by-post. Your match starts, you have a week or two of activity, and then it's over. If you're busy IRL, you can skip the next round without hurting the game as a whole. If you don't like the game, you can quit without hurting the game as a whole. Nothing that any one player does prevents, or even significantly hinders, the game's continuation, so you avoid the "one bad apple spoils the whole game" problem that's endemic to more conventional PBP adventures.