>>53257578I assume that they didn't have enough space to remake the entirety of Johto, plus as
>>53257582 said remakes of the Johto games would not have been as warranted/highly requested/highly hyped.
Sevii Islands were a great way to introduce some rarity of Johto mons into gen 3 as well as expand on specifically the world of Pokémon at the time since the route creation and Pokémon distribution was more easy to make well thought out than copying and pasting from Johto and hoping for the best. Sevii was also a good way to introduce mechanics not in Kanto, like breeding or having a Battle Facility (fyi there were two eReader buildings here but they were both cut in the international releases, which sucks because the Trainer Tower sounded awesome).
And if it's worth anything else, all of those islands are actually a part of the Tokyo prefecture which is the pretty much the area within the region of Kanto, so they most likely got added in to get the entirety of the prefecture or at least an interesting part of it.
