Current conventional wisdom is that after Iowa, Paul will leave the "top tier" of the Republican race. He has always been ignored by traditional media and this is the highest hes ever risen, so there is a possibility he may find the staying power.
To get to the meat of your question: why Paul now. Paul has always been a solid choice for people dissatisfied with the traditional Republican party. Many of his libertarian views speak to often ignored American ideals, and he is willing to tell it how he sees it, not the political-speak whitewash heard from everyone else. His supporters even include voters who don't normally support the GOP (hes the Republican with the most significant youth following and I've met many people who would vote Democrat-or-Ron-Paul). Hes showing strong now because the electorate has started to feel (like Paul supporters do) that the rest of the GOP field right now is in pretty bad shape.
There has been a sense of inevitability to Romney picking up the nomination, hes been near the top of all the polls since he entered the race, but it seems voters don't want to be stuck with him. They've been scrambling around and sending a long line of candidates to the front of the polls, and inevitably once they get the spotlight they make an ass of themselves. Then the voters send someone else to the top of the polls. Right now is Paul's turn there (at least in Iowa). It remains to be seen if he can stay there and defeat Romney, or if once voters get to know him better his numbers will start falling.
edit: Or is Iowa special? It is to Iowans, and any pundits who want to act like Iowa will be a bellweather of the country. Really, they just go first. The next races will be in different states, and after more time, and more news cycles so Iowa is not necessarily indicative of anything but the current opinions in Iowa.
Winning Iowa, or at least winning expectations, correlates with additional fundraising and additional support leading into the following states: NH, SC, FL, NV. A winner is able to build up momentum into those states and have the chance to continue a successful campaign (Obama for America 2006-2008). And to be honest, Iowa and New Hampshire both are special. I've been to both states countless times and the people are fervent in understanding today's issues and the candidates' stances on those issues, more so than any other state in the country. But you are partially correct with your final assessment, winning Iowa does not guarantee a successful campaign run.
To get to the meat of your question: why Paul now. Paul has always been a solid choice for people dissatisfied with the traditional Republican party. Many of his libertarian views speak to often ignored American ideals, and he is willing to tell it how he sees it, not the political-speak whitewash heard from everyone else. His supporters even include voters who don't normally support the GOP (hes the Republican with the most significant youth following and I've met many people who would vote Democrat-or-Ron-Paul). Hes showing strong now because the electorate has started to feel (like Paul supporters do) that the rest of the GOP field right now is in pretty bad shape.
There has been a sense of inevitability to Romney picking up the nomination, hes been near the top of all the polls since he entered the race, but it seems voters don't want to be stuck with him. They've been scrambling around and sending a long line of candidates to the front of the polls, and inevitably once they get the spotlight they make an ass of themselves. Then the voters send someone else to the top of the polls. Right now is Paul's turn there (at least in Iowa). It remains to be seen if he can stay there and defeat Romney, or if once voters get to know him better his numbers will start falling.
edit: Or is Iowa special? It is to Iowans, and any pundits who want to act like Iowa will be a bellweather of the country. Really, they just go first. The next races will be in different states, and after more time, and more news cycles so Iowa is not necessarily indicative of anything but the current opinions in Iowa.