I know that for people with multiple reptiles prefer to keep them in bins in an organized rack than in a traditional tank, just for cost and cleaning ease.

But I always wondered, why is it that it's acceptable to put a full grown bp in a tub about 2 ft long, 8 in across, and 4 in high (That's a total guess), but it's pretty much demanded that they not be kept in any tank smaller than a 30 gal?

This isn't me complaining. I honestly want to know. It could save me a lot of hassle in the future.