Okay, here are my reasons. Some of them may be superficial though.
1) I like the ease of getting back into the boat when there is a proper ladder. I'm usually tired after a dive and struggling to get in the boat is not something I look forward to.
2) The dive boats I've been on all had a covered section. I like having an area on the boat that is dry and out of the sun (okay I'm fair, I burn easily), wind, rain, spray.
3) So where's the head on a zodiac?? It's easy for guys, they can stand and go. Who wants to hold my drysuit while I hang my butt over the side??? (not happening!)
Upon stating these points, I have to admit I have never been diving from a zodiac or a sailboat (and I don't know what a multi hull is). I've only been diving from diveboats and once from a 14ft aluminum boat in the DR. I did watch some divers try to get back in their sailboat and it was waayyy to much work!
Diverlady