Why are romani people or gypsies despised in Europe so much?

Same reason the Irish Travellers were hated for so long. They were a poor semi-nomadic minority, so they often turned to crime for survival.

A large majority of them is incompatible with the rest of civilization. They live in slums in terrible conditions, lacking education, lacking basic hygiene habits which results in epidemics of hepatitis and other diseases, they often abuse their children, too many of them have a problem with substance abuse, they are notorious for stealing, throwing rocks at cars passing by, assaulting healthcare workers that were called to help, some of them are truly asocial in a way that they have no problems to assault the elderly for as little as 20 € and they often use children to commit crimes because children aren't criminally liable.

For being mobile and capable of understanding many locations. For being bothered inside and outside the true towns and countries of true folk.

People who live on the fringes of society and are nomadic rathrr than settled, will always be cast as scapegoats by the majority.