The reasons are thus: No one loves Romney. The establishment picked him because he's a good looking public who speaks well. However, his previous stances make him look like John Kerry 2.0 and his current stances aren't that much different from Obama. Plus, you eliminate one of the biggest hammering points to rally against Obama on, his unconstitutional healthcare plan that Romney helped implement in his own state.
Newt may be intelligent, but he is a walking character flaw. He just comes off as a lying, slimy, snake in the grass. Plus, he was formerly pushing for health care reform, cap and trade, bailouts, and Dodd Frank. Sounds real conservative to me.
Santorum is an idiot. He's a big government Neo-con whose economic policy isn't bad (short-sited maybe, but not bad). Too bad no one's ever heard it. The idiot can't stop talking about social issues when the election needs to be about the economy, jobs, taxes, spending, and healthcare. This is especially true when the most fired up part of your base is the tea party which has said to hell with social issues, we need tax, spending, healthcare, and economic reform.
And then there's Paul. He has the ideas. Some are absolutely necessary (end the fed, slash spending, end foreign wars, true free trade, end foreign aid, ect.), but his delivery is poor. He's all about substance, but he doesn't have the style to win over America's dumb electorate. It also doesn't help when the media treats you like room 13 in a hotel.
Paul's what we need, but Romney's what we're getting. Actually, 4 more years of Obama is what we're getting. If Romney is nominated, republicans will stay home in Droves and Obama will cruise to an easy reelection.