Any time a politician faces pressure to do something he doesn't want to do, there's a calculation involved about the arc of the story and the cumulative effect of the two courses he could take. I can take the slings and arrows of the moment and hold out, in the hopes that the story will go away, or I can succumb and hope that by getting the pain over quickly, the damage will be minimized. The conventional wisdom has become that any time there's damaging information about you, you have to get it all out as soon as possible, and there are certainly plenty of cases in which a politician didn't do so and ended up suffering both from the information itself and his initial stonewalling against releasing it. But that need not always be the case. Mitt Romney may just have bet correctly that he could stand firm against releasing his tax forms from any year before 2010 and get away with it.
We need not go over all the possibilities of what might be in them, but suffice to say it's something Romney really, really doesn't want people to know about. So he made a decision to just refuse, and he has certainly paid a cost so far, in a lot of negative coverage and fodder for Obama television ads. But in the end he won, sort of. Democrats will continue to needle him about it, but I'm guessing that the intervention of a major news event (the mass murder in Colorado) provided just the distraction Romney needed to get people to stop talking about his taxes. Unless something happens to bring the issue back up, reporters will quickly start treating it like a settled question that no longer needs to be discussed at length. And we're never going to see them, which for Romney was apparently the most important thing.
Now, I think the idea that you could become president of the United States without sharing detailed information about your finances beyond a year or two with the American public is absurd. When you want to occupy the highest office in the land, there are some kinds of privacy you're just going to have to give up. But as has happened so many times in the past, Republicans realized that there are almost no rules in American politics, only norms. And once you decide a norm doesn't serve your purposes, you can just ignore it, and if you've got your party behind you, before you know it the norm will cease to exist.
There is one thing that could revive this discussion and resume the pressure on Romney to let people see those previous years' returns, however, and that's the release of his 2011 return. Romney has said he'll release them when they're ready, and I guess they aren't ready yet—that team of accountants he employs sure does take its time. I wouldn't be surprised if they become ready some time around the second week of November.