Makes no sense. So if someone who missed out on an old expansion (or didn't fully complete it) and wants to level an alt through it, they should first complete all the quests on a max level char before being able to do so? Probably the most retarded idea ever.
Now OT, they probably restrict it to not overwhelm new players and have them quit in frustration, it's not exactly the friendliest of systems.
Plus that the newer expansions have the new, more flashy models, which are probably more likely to keep new people and a very streamlined experience with little to no exploration being required.