Betazed was probably named just like Delta was: A code name given to it by the human explorers who made first contact. Similar to the Deltans (who are actually from 'Dhei' of the star 'Lta', and the humans who visited them simply shortened it to 'Delta' - thank you Christopher
), the Betazoids' own name for the planet is probably completely different. I'm guessing whoever made first contact with them simply gave it a letter combination, i.e. 'Beta' and 'Zed'.