What is it that guys like about eating girls out while on her period?

You'd have to ask the individual guys.

In general heterosexual guys (or guys who like to give oral sex to females) enjoy giving oral sex because it is pleasurable for their partner so in turn pleasurable to them. During menstruation women can use options like tampons, menstrual cups, softcups, diaphragms, sponges, etc. to stop blood from leaving their vagina so oral sex during menstruation is no different to at any other time in the cycle. Some men enjoy to taste and smell menstrual blood as part of a sexual fetish.