The ultimate reason is that not enough people are going to schedule hours out of their day to play a single game online which may or may not end with your opponent quitting halfway through, or which leaves you open to engine abuse.
More specifically, the problem is that a classical game of chess takes hours, and that means that if two people want to play, then they each have to believe that the other party is just as committed as they are. In-person, this is easy: you've both shown up, you say hello, maybe you already know each other, there's good reason to expect a post-mortem discussion afterwards, maybe there'll even be time to go for drinks. Online, it's nothing like that: you get "paired" with a random stranger, probably they aren't talkative, you have no idea what they're doing on the other end, you can't tell when/if they're at the board, and since you aren't both in the same location there is no feeling that you've both decided to stay until the end of the game.
I could spend an hour playing a horrible game like that, or I could spend an hour playing 6 games of blitz, which doesn't suffer from any of those problems. Which do you think I'm going to choose?
(Which one is better for my chess is a different, irrelevant question.)
More specifically, the problem is that a classical game of chess takes hours, and that means that if two people want to play, then they each have to believe that the other party is just as committed as they are. In-person, this is easy: you've both shown up, you say hello, maybe you already know each other, there's good reason to expect a post-mortem discussion afterwards, maybe there'll even be time to go for drinks. Online, it's nothing like that: you get "paired" with a random stranger, probably they aren't talkative, you have no idea what they're doing on the other end, you can't tell when/if they're at the board, and since you aren't both in the same location there is no feeling that you've both decided to stay until the end of the game.
I could spend an hour playing a horrible game like that, or I could spend an hour playing 6 games of blitz, which doesn't suffer from any of those problems. Which do you think I'm going to choose?
(Which one is better for my chess is a different, irrelevant question.)