The same reason he was stopped by Angel Manfredy: Because in spite of his heart, punch, and exciting style, Gatti just wasn't that great.
Because that was around where Gatti peaked, where his ceiling was. Hence his mixed results and close fights against plenty of solid, but not world class, opposition.
Ivan Robinson had limited power but was quite skillful and had a great amateur career. Gatti was strong , had solid punch but was very crude. And like fellow 140 lb fighter Ruslan Providnikov found out against top opposition heart is not enough to conqueror slick fighters.
I agree Gatti just wasn't that good. His popularity and good matchmaking made him to appear, on the surface, to casuals, to be be better than he was. Robinson beat him by being the busier guy - he threw well over 100 punches/round
That's the flat out truth .. at 130 he was at his best and even at that weight not great .. he was diced up my Manfredy, outboxed twice by Ivan, blown out by Oscar .. Gatti was a ballsy fighter with a bunch and a ton of heart but really was a carefully matched and promoted fighter after the Oscar fight .. Mickey Ward and those bouts .. two colorful journeymen of the right color ..