The why is simple, interesting and depressing. In the political process, there's no way for the ordinary voter to register the intensity of his preferences. A bare majority of anti-smokers may support smoking bans but smokers who intensely value their right to smoke have no way to politically register their intense objections. They have just one vote that is just as good as somebody else's. It's different in the marketplace.
People can register preferences.
The depressing side of why such an omnibus bill would not pass has to do with a broad contempt for liberty. For example, there are many pro-abortionists who'd support all manner of restrictions against cigarette smoking and handgun ownership but they'd never support the omnibus bill. The reason is there would be a restriction on the liberty they intensely care about. For their part, anti-abortionists wouldn't support the bill because it would place a restriction on a liberty they highly value such as prayers in school or handgun ownership. The particular examples I've chosen aren't as important as the fact that we tend to value our freedoms and tend to trivialize and trample upon those of others.