If Darwin and Wallace independently developed theories of natural selection and jointly presented their findings, why is Darwin the one associated with it?

The way it played out was roughly this:

Darwin labored over his work mostly secretly for years and years and years. He shared it with a few friends. But he was afraid to publish it until it was "just right" — and maybe he was just afraid to publish it in general.
Wallace meanwhile works on the idea and starts to shop it around. One of these people he contacted about his theory was (not entirely coincidentally) Darwin.
Darwin sends this on to some of his more famous science buddies and says, more or less, damn it, I guess I'm beaten, what a loser I am, etc. etc. (Darwin was prone to this kind of thing.)
Darwin's buddies say, All is not lost, Chuck! They hastily arrange a joint announcement of Darwin and Wallace's theory. They are trying to preserve the priority of their friend, who they know for a fact was working on basically the same theory in secret for years and years.
The joint theory is announced publicly, by Darwin's buddies (Darwin is not the sort for public appearances). It gets very little notice — the whole point of announcing it was to secure Darwin's priority, while making a nod towards Wallace being a catalyst and smart guy.
Wallace was still in the Malay Archipelago so he doesn't get to say much about this — he doesn't find out until after months the fact. But he says it sounds good to him when he finds out.
Darwin, now suitably pushed, went on to write and publish On the Origin of Species, which he considered only an "abstract" of his real work (the real work being so large and ungainly that Darwin never wrote it).
Wallace shows up after this and becomes pals with Darwin. He never really pushed the priority question at all. Both continue working on evolutionary theory among other things.

Darwin was better connected that Wallace (something that Wallace profited from, which may be why he never pushed the priority thing). Darwin also churned out several works on evolution, and did quite a job of arguing in favor of it. Darwin became canonically associated with natural selection as a result, and his friends, relatives (e.g. Galton), and acolytes generally enjoyed crediting Darwin with the major work. It should be noted, again, that Darwin basically did not do public appearances or argue his theory personally — he was pretty reclusive and operated by sending letters and writing books. I suspect this made him an easier figure to adopt as an originator than Wallace, who became much more active over the years and occasionally did things that put him at odds with his other colleagues (like get very interested in spiritualism).

Ultimately all attributions of credit in the case of priority questions are tricky historically. In the case of Darwin the canonical answer is he did it first, and best, thus he gets credit. The fact that he did it in secret, did not get it done until Wallace appeared (and probably would never have published on it if Wallace had not written directly to him), and so on, makes his case trickier, though, depending on what norms of scientific conduct and priority you want to adopt. There is a not insignificant "Wallace was robbed" sentiment amongst historians of this period, because Wallace really does get out-maneuvered, his consent to this second-banana status presumed, and Darwin really did have many more resources (monetary, social, and scientific) at his disposal in order to secure his own priority claims.

Fun questions to consider:

If Wallace had just published, and not consulted Darwin first, would we still think of Darwin as the originator or just a weird guy who had the same idea but never did anything with it?
Do we place too much importance on Darwin himself, given the fact that there is good reason to believe that even if he hadn't existed, or died earlier, or whatever, that natural selection would still have made its debut roughly when it did? That is, are we making an implicit counterfactual argument by making Darwin into such an important figure in the late 19th century, as if things would be significantly different if he was gone?
If Wallace had not been such a small fry, and at such a social and geographic remove, would Darwin and his buddies have behaved the way they did?
If Darwin had just pretended Wallace's article had gotten lost in the mail, and published his own book without mentioning it, what would we think about him?

On Darwin's priority behaviors, the Wallace story, and everything else, Janet Browne's second volume of her Darwin biography, The Power of Place is a wonderful read and gets into this stuff very closely.