Porting a game such as Super Smash Bros. and Mario Kart 8 does not take a lot of resources from the development team and it's more of layover than a original game so that they can work a true sequel.
Since these games are highly rated and extremely good games in their own rights, it makes perfect sense from a business and consumer standpoint to port these games over with all their DLC plus new content added.
The Wii U had a low user audience and game library but had some very good critically acclaimed games that were not enjoyed by many due to it's low hardware sales and it'll be even more difficult to play those games now that production of the Wii U has ceased, so it makes perfect sense for those games to be ported over to the Switch which can now be played in 1080P and on the go.
Nintendo is not forcing users who previously owned these games to buying them again but for plethora of new users that own a Switch and skipped the Wii U (including myself), being able to play these games in 1080P or portably would make it a sure buy and extremely profitable for Nintendo from a business perspective.
Not only will Nintendo sell these ported Wii U titles but these games would also encourage Switch users to purchase more peripherals such as more Pro Controller and Joy-Cons, it's a win/win situation for Nintendo.