Plus, if you remember, Patton is wildly popular and successful, his voice means something. Truman might take a hit for forcing into retirement one of his best generals during the election and that could tip the scales in favor of MacArthur. Also, I never had any intention of having him back MacArthur, he was always going to be backing Truman, so why would Truman or anyone in his government demand the resignation of one of his biggest backers?
Because everyone will be demanding it, the military does not get involved in elections. Breaking that tradition would cause a hell of a backlash. Hell there are problems when retired generals back a candidate. The military is suppose to remain apolitical and apart from such considerations in the US.
I mean you are aware that it's against US law for anyone to expresses a political opinion in uniform?