The fact that Trump spent most of his adult life outside of the Republican Party is why he is embraced by the nation.
The years 1999 to 2012 hardly constitute "most of his adult life", but your point is still somewhat valid -- he was a businessman, never a politician.
It's also worth remembering the Democrat party turned on Trump with a vengeance, once his Republican affiliations became well known in 2015.
I don't remember Republicans ever attacking him like that when Trump was a Democrat.