ball pythons and corns are the easiest to obtain from the wild which is the cause for the popularity among other things (size, morphs, longevity, temper) boas dont have the same range in color mutations yes many different locales and many different sizes but bps are almost all same size and all from one place.
retics too big, burms too big, carpets a definite contender if it was easier to obtain them

i would love it if australia changed their laws i would go hunt down a whole lot of them and bring them back if i could then again i've been to australia once and swore if i go back i wont ever leave, beautiful country!