Two things, and two things alone makes a species popular:

Cost and availability.

Want to know how to find a popular species? Look on craigslist for people who are giving away pets. Bearded dragons, ball pythons, green iguanas and savannah monitors is virtually all you'll see. Why? Because they're cheap and they are at Petco/Petsmart. And yet people who impulsively buy them for their kids or themselves have no idea what it's really like to take care of them. On one hand I applaud them for trying to give them up to someone who can care for them but you know that nine times out of ten one ignorant pet owner is just going to give their pet away to another ignorant pet owner. They're destined to be on craigslist every three months until someone can rescue them.

There are breeders of monitors who refuse to breed savannahs. Because they are bad monitors? Nope. But because they don't want to be a part of this cycle of disposable pets. Most people are good people but when it comes to pets they don't take anything but cats and dogs seriously. They have this mentality that if it's small and lives in an aquarium it's disposable. It's disgusting.

Most people who buy reptiles, amphibians and other exotic pets have no business doing so.