Because he had decisive control at Palace. This is how UEFA view it, as ludicrous as it sounds.
Supposedly he would be able to go into the Palace changing room and influence a game to benefit another of his clubs.
He could only get out of this by recusing himself from both clubs.
Interestingly, if Textor had put his Palace shares into trust and not his Lyon shares then Lyon would have been relegated to the Conference.
My personal feeling is that Textor knew this all along but took a calculated risk that Palace wouldn’t win the Cup hence why he had an, “oh s***” moment when Palace did win.