Because Sun/Moon has noticeable lag on old 3DS relative to New3DS in several places

Also loading is way faster on New 3DS, Old 3DS suffers from delay upon move selection in intense battles because it seems like the AI takes forever to run and Old 3DS can outright crash on one of the fights so strats are slightly different between the two there.

Out of curiosity, what if I did it using my 2ds? Would it get it's own separate list or would it be together with the old 3ds?

Sun/Moon have noticeably more lag in certain areas of the game on old3ds than new3ds, seen just by looking at the times for each. Gen 6 doesn't have this lag problem, so they aren't split. The New3DS family (new3ds, new3dsxl) has better hardware than the old 3ds family (3ds, 3dsxl, 2ds) and can handle much more in terms of graphics quality and stuff like that. Sun/Moon weren't really designed for the old family, but are still compatible. Those were just the consequences of that.