The major problem is that the corruption schemes were run by his party but they never really managed to prove his own personal involvement with it.

When they did "prove it" (and had him arrested for over a year) it was by a judge who was considered partial by the Supreme Court and had a lot of cases thrown out on partiality and something about how it should have been judged in another place.

No one doubts his party was involved in corruption schemes. They were. So we're their allies and the whole thing.

The main problem lies in optics: What was Lula's role in these? Why was his successor (Dilma Roussef) impeached? He and his party have never really addressed the issues so people have issues with that.