I think this is a major problem with online dating - maybe the major problem.
It's like being a kid in a sweetshop.
I know some daters are married, some get scared and don't want 'commitment', some people go back to an ex or something and that's why they drop off the radar. But it's the constant barrage of new potential opportunities which are in their face even while they're dating that causes most problems I think.
If you compare it to the real world, lets say you're at a dinner party, or a bar, or a friends barbecue, and there's a cute stranger who's smiling at you. So you get chatting. You like him/her, you exchange numbers, maybe a quick snog if you have absolutely no shame :) And you date, and you date, and you move in, get married, have kids etc. Maybe.
But, if you're at that same barbecue and while you're having flirtatious glances with him there are 25 other nice looking blokes winking at you, and 25 other women smiling at him, and when you chat to him, he's chatting to the others at the same time too, and you're chatting to some of the others guys. Some of whom are slightly better looking, or funnier, or taller than the one you were just talking to. But then you find out one of them doesn't like opera so it's doomed because you looove opera. But - one of them is slightly better, or at least asks you out first.
And when you go on a date with him to a nice restaurant you look out of the window and 3 or 4 of those women from the barbecure are still there smiling and having a conversation through the glass with the guy about their date next tuesday. And at the other window there are 4 or 5 guys looking at you, waving, and one of them is reeeeally lovely...
Is it any wonder the failure rate is high?