Because some people have an ongoing argument with Bush, have personally declared Reagan an icon airbrushing away any flaws he may have had, and think if a Republican comes out endorsing Reagan and shutting Bush out they have a champion. Finally, a REAL Republican with the guts to stand up to Bush yada yada...
This is what they want Republicans to do and they've admitted it in so many words. They want Republicans to defeat Bush and pass THEIR narrowed agenda without thought to the consequence if the President goes down, WE go down, and NO conservative agenda stands a chance at passage. It'd be the open doorway for the RINO's to seize power, not vice versa with conservatives in charge.
Many of these people tried to convince us not to vote for him in the last election. They lost in 2000, and 2004, and they haven't gotten over it so this is their latest attempt to defeat him.
Just as the "purists" in the Democrat Party want their leaders to break, not with a leader because they have no leader, but with the last thread of sanity that states supporting vocal solidarity with the terrorists is just 'not done'. They want a prominent Democrat to be their champion. Their Cindy Sheehan, only in elected office, and Conyers isn't good enough. It has to be someone like Hillary Clinton.
In my experience, someone that states they are a Reagan Republican is usually anything but. They are using his reputation to put them in office, or attempt it. A real conservative stands on their own ideology and has no need to offer Reagan as a reference if they've the record to back up their conservatism.
As well, if someone claims to admire Reagan, denounces Bush, and praises McCain, this should send up red flags to anyone that isn't blinded by an agenda against Bush.