Aesthetics, mostly. Tanks tend to look a bit better than tubs. Although tubs I get are pretty clear, and if you want, you can get the taller ones and decorate it as a naturalistic enclosure. You can also use a heat emitter to supplement if you can't get your ambients up, although I'm sure with a big enough tub you could modify it to handle a light without melting. Tanks are also pretty popular because that's what's pushed by petstores I'd much rather have a tub though. They're light, easy to clean, and work quite well for my purposes. But to each their own.