Why are nearly half of Republicans dissatisfied with the current crop of candidates?

Because they won't risk any golden candidate this election, they pretty much know that beating the incumbent (Obama) is going to be way too difficult of a task so at this point why not let Romney run for President who will not win but at least he'll win more electoral votes than McCain did hence making it a closer election and eroding some of the Dems' strongholds and at the same time they get 4 years to really groom up a good charismatic contender next time around such as Marco Rubio, John Thune, or Aaron Schock. It's called strategy and it's not the first time it's been done, see John Kerry 2004, Bob Dole 1996, and Walter Mondale 1984.