Why is he even focused on the NFL? He gets more mad the darker the person is... And does he not know white people love Marshawn and Steph Curry? Someone should tell him this... He's not racist, but most of his base is and he uses this crap to hype them up...
May be a combination of racism and his resentment towards the league that rejected his bid to become an owner. Add in the very real effect of his having gotten a big boost from his base while throwing out a line about it in a speech. Some of Trump's so-called values started as throwaway lines that happened to get a big response. (This is exactly how "The Wall" got started).