It’s a great question. And it is not answered in this somewhat lengthy Scientific American article, but the article is worth reading for just asking the question.
What it boils down to is this, African clawed frogs were among the first frogs to be diagnosed with frog-killing chytrid fungus infections. The are kept by the hundreds when they are shipped around the world as research animals and cheap pets. Even one infected frog introduced to the distribution system has the potential to spread the fungus around the world — particularly if the frogs are released into the wild.
African clawed frogs do not show symptoms of the fungal infection. Neither do bullfrogs, which have their own global distribution network. (They are food.)
There are lots of links in the article. Don’t ignore them. They bring you to journal articles and other supporting information.
Photo: African clawed frog by Chris Brown, courtesy US Geological Survey