Because popular vote is not what the Republican Nomination for President is based on in most cases. It's based on the number of delegates the nominee has accumulated. Delegates are supposed to be representative of the consensus from the area in which they were chosen, but in most cases the law does not require them to vote for the nominee which they were actually chosen to represent, which means that some delegates might actually vote for Paul at their convention, despite being chosen to vote for Romney. A large number of delegates chosen so far are not yet committed to any candidate at all - news organizations are reporting Romney with a large number of delegates, while Paul is reported as having way fewer, when in actuality there is a mass of uncommitted delegates nearly as large as Romney's total floating out there who will probably vote for Paul if the choice comes down to Romney vs. Paul. That's a situation Paul supporters are working hard to create.
Also keep in mind the substantial, but unverified, evidence of vote rigging in pretty much all the caucus-holding states. Caucuses are run by the political party of that state, in this case the Republican party, not the state government like primary votes, and are generally immune to actual government oversight, therefore their fairness has been questionable throughout American caucus history. The Republican party counts the votes at multiple levels, eventually ending up with a whole final vote. They're in charge of the numbers, and it's easy to imagine them manipulating the vote to favor the party's preferred candidate.
Basically, the Ron Paul organization is relatively underground and isn't being reported by Big Media. As long as we can keep the money flowing to his campaign to pay the workers and pay for advertising, I'm confident that jaws will drop when the time comes.
