As I understand it it's basically a combination of disillusionment and their material conditions. Trotskyism was a more palatable option for young people with leftist sympathies but too much Cold War conditioning and too little theoretical grounding to side with the Soviets against their own country, so as they grew older and richer, there was no perfect magical imperial core revolution coming along for them to tag along to, and they lost whatever principles they once had they took the framework they'd gotten from trot theory and reapplied it to the ends that a comfortable upper/upper middle class American would want, namely nationalist and imperialist goals.

Or to put it another way, if someone was sectarian against the USSR and their material interests were those of a comfortable white American beneficiary of empire, that's a whole lot of pressure to embrace American nationalism and go all in on the empire shit.