Both bagged and ex-ground frangipani trees have their advantages.
Bagged trees can be moved around and transplanted with minimal disruption to the roots. They can be transported standing up so even a loosely tied rope will make sure they stay upright. Freight space is calculated by floor space so an upright tree will cost less to transport than a tree on its side.
Frangipanis usually grow faster in the ground so bigger frangipanis can be sold more cheaply. If the frangipani tree is too tall to stand up in the delivery truck and it needs to be transported on its side, a larger cheaper ex-ground specimen might be the better option.