Why personal hygiene is important for an interview?

It's because people look for anything that might tell them what kind of person you are. Whether you have good manners and good morals, are clean body-wise(which implies that you might be a clean/neat/organized/etc person, and that you might also have a clean & neat home), etc. It also tells them that you might be responsible, considerate, respectful, mindful, friendly & professional-looking, etc. Showing good hygiene makes them feel more comfortable around you and want to do business/socializing/etc with you. Good hygiene seems to be a requirement for a lot of people in this world today, Plus, a lot of people are very picky about who they hire, what things they can tolerate/deal with/etc, who they hang out with, etc. Many people are also judgemental about certain things. Everyone is different and has different standards, expectations, toleration, levels of pickiness for hiring/etc, opinions, etc. But mostly, it's just common courtesy to show good hygiene(nobody wants to smell like grass, sweat & musk, dirt, hay, old socks, etc); and it, like many things, plays an important role in whether or not you'll be hired by most people.