1. National businesses in many cases (though not all) want uniform regulations to ease doing business. So they lobby for federal preemption of state regulations. For example, car manufacturers don't want to have to meet 50 separate safety codes in order to sell cars nationwide, so they successfully lobbied for a federal code with express state preemption. Advocates of tort reform often have similar motivations: a key proposal of tort reform is to federalize product liability, because manufacturers dislike the current system where litigation will always end up being initiated in the most plaintiff-friendly county of the most plaintiff-friendly state.
2. The last time state vs. federal power came to the point of an outright test of strength, the federal government won. And what's worse for the states, its victory has in retrospect been very popular, which has done quite a bit to boost the popularity of federalization and stigmatize the slogan "states' rights". JFK sent troops to Alabama in 1963, removed the governor's control over the state national guard, and imposed federal law by force; and nowadays most people think he did the right thing.
3. People move around a lot, which makes it increasingly impractical to deal with things like social security or Medicare at the state level, when you might be born in one state, work in three others, and retire in a fifth. (This one is becoming a problem in Europe, too, and will probably lead to some kind of EU-standardized "portable pension" scheme in the medium-term future. An EU-wide health card has already been created, though its terms are not yet standardized.)
