With Splatoon 2 it's probably because it's a full sequel that's being released rather quickly after the first game, which itself saw a ton of updates for a long time after release. Between having a full-fledged single-player mode and a fully featured multiplayer suite something had to give to get it out this quickly.
Mario Kart had already gotten its last DLC pack before Splatoon was first released, giving the developers more time to get the Switch version polished than Splatoon's developers had for the sequel. Plus it was an enhanced port rather than a completely new game. So outside of the battle mode arenas there was nothing really new that would have taken their time. It's completely understandable why MK8 got to be updated to 1080p while Splatoon 2 is still at 720p.







