The why hardly matters. Only the consequences. You can definitely argue that FDR failed to cement the more progressive programs (fully employment through public agencies, public control of finance and agriculture, a long term peaceful coexistence with the Soviet states). And for that reason, he was a kind-of failure. But I would argue putting the weight of the world on one man's shoulders is deeply unfair. FDR took US policy as far as he could. Then it was Truman and Eisenhower and their lackeys who fumbled the bag (or capitulated to corporate interests deliberately).

His strategy to contain the CP was to neuter the party by bringing it into the Democratic party fold, alienating their most militant members, and slowly squashing their agenda.

The Democratic Party, as a whole, has a vested interest in neutralizing rival movements and harvesting their members. That's not a strategy FDR invented or pioneered. Neither was the DemSoc liberalism of FDR incompatible with a more Reform Oriented American Communist Movement. The strategy worked in large part because American Communists saw FDR's outreach to Stalin's Russia and Mao's China as a positive turn foreshadowing a real global movement.

I might argue that Stalin's "Communism in One Country" and Mao's failure to open China up until Nixon, thirty years later, that did more damage than FDR's liberal-washing of Communist organizing efforts. I could easily argue that the Truman/Eisenhower Cold War was what ultimately did in the American Communists. Socialists couldn't uproot Hoover from the FBI or unseat McCarthy from a strong union state like Wisconsin or keep guys like Nixon or Kennedy from worming their way into the upper echelons of the US government on a wave of mafia money.

At some point, you have to acknowledge the failures within the leftist organizing movements that happened in the US. Deng and Khrushchev and Ho Chi Mein and Kim Il Sung didn't collapse in the face of these problems in their home states and they all had it much worse.