A combination of time and power difference. Many third party developers probably didn't want to gamble on the Switch before they saw how it would sell. Now they're likely more inclined to have their bigger titles also end up on Switch when possible. We should hopefully see more of that in 2018.
If a game is too difficult/costly to port over to Switch, it may be better to just move on to the next game, and this time develop the game from the start with Switch in mind.
It's also important to remember that publishers follow profits, not specs. If the Switch provides an opportunity, they'll jump at the chance, even if it seems impossible. If the Switch can run a scaled-back version of Doom, then it can run many other PS4 and Xbox One titles.
If that were true, Wii wouldn't have been overlooked by virtually every major third party Japanese franchise. If there was a new Resident Evil, you didn't expect to see it on Wii. Rather it got a port of the previous gen RE4, etc. Doom already ran at 1080/60f on PS4. There's more room for performance optimization in games like that. If a game struggles to meet 30f on PS4, it's a different matter. Especially if it wasn't designed with the Switch in mind.