Because it is Right, it is Just

I suspect that Trump noticed that republicans were easily persuaded to vote for a mentally-disengaged candidate like GWB because GWB kept referring to "family values" over and over again three years after Clinton was found to be getting blowjobs from an intern. So even though Trump's first reaction in 2001 was to register as a democrat (so he could start banging interns), he kept that thought (of conning gullible republicans) in the back of his mind, and as he got older and pondered a way to be America's dictator a decade later, he realized it would be much easier as a republican.