We know the real world reason: limited storage space means limited everything.
That is why I would love a genuine remake of Phantasy Star 1 and 2.
I do not mean it as an insult to people's fond memories but I feel that both games can really be greatly expanded without loosing what people liked about it.
(I have not played PS Adventure, but in that game, isn't it common to travel to different planets even during the time of PS II?)
At the time of PS Adventure interplanetary travel between Palma, Motavia, and Dezoris was still common, six years later after the accident in space it was stopped by Mother Brain. (BTW, one collision in space puts an end to interplanetary travel? In the thousand of years since it was first developed there must have been more accidents in space, it seems ridiculous to outlaw it after just one accident)
Personally I think the designers should have considered the idea of having Tyler offer the player + party travel onboard his ship.
It would have given the character more purpose.