Accessibility. Crickets are a good source of nutrition relative to their cost, and they can be gotten anywhere. Anyone can make a strong argument for feeding reptiworms or roaches instead, but the fact of the matter is that not everyone wants to order specialty live feeders online, pay for shipping, and have to house a large number of bugs in order to make doing so worthwhile. Meanwhile, it is possible to pick up a bag of crix on the way home from work, gutload them, dust them, and have a perfectly happy beardie.