I think much of the issue with Nintendo and 3rd party titles on their consoles comes from the fact that for much of the last 20 years they have at best been the second highest priority platform for 3rd party developers, not taking into account PC. For much of that time most 3rd party developers have been doing just fine without Nintendo, so for them to begin putting all or most of their titles on the Switch Nintendo would have to make the platform so successful that they simply could no longer ignore it as a platform.
The Wii U never reached the level of success that would have led to notable support from 3rd party developers, and while the Wii was certainly successful enough, the power parity between it and the other two major consoles meant that developing for it would have required additional resources to be spent making a separate version of each title. Even now that the Switch has proven to be highly successful, that still only amounts to 5-6 million consoles sold, which isn't a high enough number to really attract many big developers yet.
Nintendo is definitely doing a good job to set the console up for success in the long term as well, having a console that is more or less comparable with the two other major consoles, but it's not there yet, at least as far as 3rd party support it concerned. We might see a change in this around the holidays or early next year once the Switch passes 10 million units sold and becomes viable platform for other AAA developers besides Nintendo.








