Idk man something seems fishy about this. The game was recently playable at PAX a week ago and many people said it ran just find they even had playable Blightown and it was even running smoothly. The game was at 1080p docked 30 fps so unless they're trying to get it running at 60 fps which let's be honest should be able to do since it's an old 360/PS3 port I don't see why this was delayed and I have two theories:
1. Nintendo told Namco Bandai to hold off until the Nintendo Switch Online launches in September (still sucks we have to wait a year and a half into a consoles life for decent online) so they can use this game as one of it's big selling points along with Smash if it comes out in September. Maybe Nintendo was originally planning on launching Smash as the big game for the online but maybe development needs more time so they're using this instead who knows..
2. Or this may sound asinine but hear me out..maybe 3rd parties don't want to day/date releases of multi-platform titles with PS4/Xbone because they fear it will take away from PS4/Xbone sales because of the portability nature which believe it or not is swaying more and more people away from 1080p/4K resolution. They probably don't want to ruin their reputation with Sony (the recent Dragon Quest XI situation is very similar to this as well). So they release the Switch port late knowing either 1 of 2 things will happen, 1. People will wait on the Switch version or hell even double dip or 2. The Switch version comes out later and bombs but they use the "See Nintendo audiences don't want games" excuse to make them feel like delaying the game wasn't why the game didn't perform to expectations. Self-fulfilling prophecy.