I know this is about personal preference, but there also has to be some more reasoning behind these choices. I mean, this isn't picking different flavors of ice cream; these choices on what animals to keep are usually long term commitments.
I guess it might also stem from what excites the given individual the most about reptile keeping. For me, I like watching reptiles eat, especially snakes (although the meanest animal I have is the turtle, who is a sadist to crickets (ha)) So when I have different types of animals I get watch all different kinds of eating behaviors. One of my favorite examples is with crested geckos, because they'll lick crestie diet right off your finger and it feels like a mini-cat tongue.
Then again, I'll go to a reptile expo and see a table with an assortment of ball morphs and wish it was my birthday all the time forever.
I would love to have a monitor and watch them pile through bugs though. Then again, a chameleon would be spectacular to watch eat.
But that's my take; people have the animals they have because those animals best match their main attractions to the hobby.