Why we say Happy Holidays instead of Merry Christmas when addressing a diverse audience we don't know personally:

It's not because we have something against Christmas or Christianity or Jesus or whatever.

Christmas is just one of the major holidays that takes place during this time of year. The others are Hanukkah, Kwanzaa, and New Year's Eve/Day. 

Some people do not celebrate Christmas so it makes no sense to wish them a Merry Christmas. Also many people celebrate the new year, and saying Merry Christmas does not include New Year celebrations.

So without knowing what they celebrate at this time of year, it makes more sense, and shows that you're not egocentric, to wish them Happy Holidays.