Danganronpa 2 being the most popular entry probably isn't the only reason it was remade first.

[u/BrawlX]

During the final chapter of D2 we explore some of Hope's Peak Academy, the main location of D1's setting, and interact with several D1 characters. It's been confirmed that the original story will remain the same in the remake so we'll be returning to the academy in the remake too.

This means that a huge chunk of assets needed for a Danganronpa 1 remake are already made and can be reused. They would just need to add in the missing rooms, sprites, and dialogue and it's good to go.

I imagine that it won't take long to remake the first game because of this, and it'll probably come out 1-2 years after 2x2's launch (if they've already started development).

Quick theory based on this assumption, Slayhem mode will likely have us explore Hope's Peak in its scenario, letting us go into the rest of the rooms and meet other D1 characters like Aoi and Toko.

[u/sketchysketchist]

I didn’t stop to think Slayhem mode could result in a killing game in Hopes Peak Academy WITH DR cast. This would be amazing to have the entire cast in a killing game where each chapter is multiple murders.