The top GOP candidate Jim Gibbons chose not to run against Reid. He preferred to keep a safe house seat. Reid also got a lot of support from GOP business interests. Go figure.
In addition, top GOP candidates are just too afraid to lose-they're cowards. If this were not the case, Heather Wilson would be running against the incumbent in New Mexico, Caputo would run against Byrd in W. Virginia and potential giant-killers Romney and Rudy would be running in N.Y. and Mass. The only exceptions to this rule are John Thune and sadly, Katherine Harris.