It's because of the fact that they had PS4/XB1 in mind from the getgo and adding the switch on top of that when they also have to update those versions with future updates and get ready for their next game would be stupid. UE3 isn't like UE4 on the Switch, and I doubt they'll pay to hire anyone to make a late port at this time when it wouldn't be worth it.
Of course from what I gather, they might have been able to try and have it work out if Nintendo gave them a dev kit pre-switch launch, but it was no guarantee. They have one now, but it seems like it'll be used for a future title instead of for this game due to the timing.
Porting's all about timing. If you're held back by a lot of other things and don't really have to owe anyone the game (it wasn't a kickstarter goal for a Nintendo system) then why bother taking a risk if you already made your money? Nintendo fans tend to be very selfish when it comes to ports on the switch, and sadly it seems a lot of them simply don't know that 90% of late ports = risky disasters.