Given that in the latest interview with Matt and Melissa, they both said that CoH was making MORE money when they went to F2P, I can see why.
Well, that's not the reason, that's just the sign.
The reason is a bit compound. For one, there are so many games out there, so many MMOs. It's hard to spend any money on a game you may not like. A F2P model drastically lowers the entry friction, making it extremely easy for anyone to just download the game and try it out. This brings in a lot of more players than you would.
A second point is commitment. Once a player is in, he is normally required to keep paying his monthly fee. It's a bit of an all-or-nothing. With a F2P model, the player can decide to only pay certain months, or micro-manage what privileges he is willing to pay for. One of my best friends in game used to cancel and return all the time due to RL issues. I bet had he have a F2P model at the time, he would never have forced himself into these windows. Eventually, during one of these windows, it seems he just never came back. In short: RL forces you to quit for a bit and once you quit it's more likely you may never return. With F2P you are rarely forced to quit, you are still there the day you are able and willing to pay for something.
Another killer of MMOs can be small communities. If you have empty servers, people stop playing even if they love the game. Even the stingiest of F2P players, the ones that never spend a cent in the game, are actually contributing to the success. By filling servers with team-mates for those that are willing to pay, they make sure that those paying customers never quit out of seclusion.
And then, an issue that comes up with this: once too many engage into F2P models, it becomes even harder to convince anyone to try your MMO if you are charging money up front in any way (be it box, monthly fee or worse, both.) Right now, launching a paid MMO is crazy and you should not attempt it unless you got a HUGE level of hype and strong IP on your side. There are just way too many free options out there for you to be able to gain any traction as a paid game.