At the time they started development they didn't really know the Switch would be a successful market. By the time they would have had confidence, the game could have not lived up to expectations and maybe they decided more ports didn't make sense. There are also issues with rights that can be very tricky especially with Disney and older games. Might have been that the contracts didn't allow Capcom to put it on any other systems, and Disney wasn't willing to extend it to the Switch at the same cost, or had a general change in policy, etc.
It is also noteworthy that the collection released in April 2017, shortly before the Duck Tales Reboot started. So, it was the perfect time to release that type of collection. By the time the Switch launched successfully and the development could have been completed, maybe the moment would have passed and the game wouldn't have sold as well.
So, of course, I can't answer the question, but there are plenty of reasons this is possible. When you get into licensing deals involving multiple companies, things that seem very simple can become very complex. Probably why I am posting this instead of playing Capcom vs SNK 2 right now.