Because it's easy to port and can sell. Or at least, that'd probably be Nintendo's train of thought.
Like I mentioned before, I don't like assumptions whatsoever especially when it comes to business decisions and I personally don't want Nintendo to have that train of thought even if that is the case.
I never said Mario Maker can't be ported.
I never even mention about that too lol. I was talking about porting "difficulties". I just don't like how we keep assuming which games won't be coming to Switch, or which ones are easy to port than the other one when no developers come out and say it.
Just take Crash N Sane Trilogy for example, somebody I am not gonna mention used to say Crash will have huge visual downgrade if it ports to Switch like back in last year December I think, thus has no reason of porting. Now we have the game and the graphics turns aren't that much of a difference... yeah...
And now you have someone from Blizzard confirms that Overwatch is "feasible" on Switch but Starcraft 2 isn't, this is what I can trust.
tl;dr
I only trust words from the developers who make that certain game when it comes to the porting to Switch feasibility and difficulties.
NSMBU is just an easier cash grab because it requires literally zero effort while Mario Maker needs motion controls and a Miiverse-like function which again, has been proven possible by another game, this time being Splatoon 2, but it's still more effort.
While we have yet to see what kind of extra content will we have for NSMBU Deluxe, I personally think it will be an awful idea to port it first just for the sake of easier development. Moreoever, sorry I keep repeating this but, I dislike making non-source back up assumptions and I highly recommend you to stop doing so even if it does sound legit and logical.
As for Splatoon 2 I got to disagree on how you consider it an easy developed game (Hope I am right on this, if I misunderstood your point please correct me). Splatoon 2's predecessor Splatoon is a Wii U game and has a sequel within two years and having extra content and still getting more, this is different compare to how we "assume" NSMBU might turn out to be... see the problem here? I think we should wait until the game is officially announced and have infos for a better discussion and comparison. I still hope they could just have a new NSMB game where is basically all NSMB games into one and last but not least, don't call it NSMBU Deluxe, the name itself sounds pretty lazy already.
tl;dr either a Mario Maker port could still happen or Nintendo's just ****ing lazy at this point.
If you are just pointing at how Nintendo is release their first party games, I can see Nintendo is somewhat of being "lazy" in 2018. But then again, you know me I dislike assumptions when comes to ports and business decisions, and we don't really know what kind of games Nintendo still have up to their sleeves. Maybe we will get something new later down the line (Whoops, I made an assumption lol, but hey there's proof last year to back up my assumption though lol)