It's not just that caucuses are inconvenient and poorly attended, it's also that more emphatic supporters have an advantage there. Where people have to get together and decide after some conversation, the enthusiastic Obama supporters might convince (or wear down) the luke warm Clintonites so that a room that starts out evenly divided ends up caucusing for Obama.

In a general election, it doesn't matter if you're head-over-heels for your candidate and I'm only slightly in favor of mine; our votes count equally. Winning primaries, therefore, might (maybe) be more of a show of general election strength than winning caucuses, if only barely.