The main problem is that the song was terrible, truly terrible (aside from being in Neapolitan and not in Italian, the rules were ad-hoc changed for him). There have been many accusations that Geolier's fans bought SIM cards in mass to vote, but I find that quite unlikely.
Certainly, many Neapolitans believe the controversies are a matter of territorial racism, but it's absolutely false—I say this as a Campanian. Even Angelina Mango has verses in Neapolitan in her songs, and she's from Basilicata.
Sanremo is Sanremo; controversies and boos are normal. It has always happened and is part of the show. I recall an edition where the jury rebelled against Emanuele Filiberto, a member of the former Italian royal family, entering the final, even throwing the sheet music
https://youtu.be/U0zFizNPkwU?si=UzASIIsv-QorjBtc