The reason Catholics are better organized than humanists is that they're official, communal, and hierarchical and we're not. The reason cults are better organized than Catholics is that they're even more official, communal, and hierarchical.

If the Pope says "Donate ten percent of your money to me," then there's an expectation that ordinary Catholics will obey. They've committed to following what the Pope says.

If you, Eliezer, posted on this forum "Please donate ten percent of your money to the Institute That Must Not Be Named", well...actually, I don't know what would happen. A few rare people might do it to signal that we liked you. But although we often follow you, we are not your followers. We haven't made a committment to you. We associate with you as long as it's convenient for us, but as soon as it stops being convenient, we'll wander off.

If you really want to get an infrastructure as powerful as the Catholic Church, you need to ask us to officially swear loyalty to you and start publically self-identifying as Rationalists with a capital R (the capital letter is very important!) You need to put us through some painful initiation ritual, so we feel a commitment to stick around even when the going gets tough. You need to make us publically profess how great Rationalism is to all our friends enough times that it would be a major social embarrassment to get kicked out for not obeying you enough. You need to establish a norm that following Eliezer's requests is so completely expected it would be strange to refuse and we'd be going against all our friends. And then you need to keep telling us about how much better off we are as capital-R Rationalists than as members of the boring old general public. Then you can start ordering us to donate ten percent of our income and expect Pope-level compliance rates.

The cultists do all of this, and the Catholics try but generally fail, which is why many Catholics don't listen to the Pope nearly as much as atheists think. If you didn't want to go quite this far, even making us pay $5 for a (physical, laminated, colorful) Less Wrong membership card would probably make a difference. Once we did that, we'd be members of something, instead of people who came to a blog every so often to discuss an interest. The brain cares a lot about this sort of thing.

[edit: better explanation below in response to ciphergoth]