>>32591936Because GF thought keeping them separate was better in earlier gens. You know, like how the Inn and Item shop are separate in most RPGs. It makes towns appear larger than they actually are, since all of them will have the same two buildings to fill the space. Add in a Gym, and you're pretty done.
But as they started making the towns actually larger, with more buildings and larger space, finding both Mart and Pokemon Center became just a chore for some players, especially kids. Look at the last region with separate Centers and Marts. You can imagine a kid not remembering where the Mart is in Hearthome or Snowpoint, because the cities are fairly large. And unmemorable. So to solve the problem, instead of making colored roofs, they combined the both.