As a kid I used to play a lot the Hoenn games, mainly gen 3 in the GBA era, it is one of my favorite regions of all time and the reason i would choose Emerald over ORAS is mostly the mystery the game gave me first playing it. Puzzles like the Regis were things that surprised me and gave me a feeling of adventure and joy, these legendaries were for the most part hidden away and you had to work your way to get them.
Some of the locations caught my attention, most notably Faraway Island and Southern Island, two locations that i know most of us or never got to experience because the requirement were ridiculous and required extra stuff, as a child I began to wonder on the Pokenav "Huh," I do wonder what this lonely island is", now thanks to emulation I can explore these locations that wouldn't have been possible and i got to say I really enjoyed the atmosphere they brought thanks to the soundtrack and how remote the places were, it's one of the reasons why I liked Emerald, the feeling of discovery and loneliness.
And let's not forget the Battle Frontier, despite the really crazy outcomes i've got in this place, i had my fair share of fun, most notably playing a whole summer trying to get all the gold symbols (even though i managed to just get all 7 seven in silver and 1 gold, someday i swear i'll try again), oh and the glitches are kinda fun (cloning).
TL;DR: Mostly it's the nostalgia factor but how vast and mysterious Hoenn in 3rd gen was and the places of interest the game had made me love the GBA games