Two main reasons - time and power.

The first one will obviously go away eventually, while the latter will remain. Some games won't be on the Switch ever due to power limitations, but it will get many of the big 3rds.

Devs played wait and see, and now they know the Switch is a beast and certainly taking over Japan very quickly, with the rest of the world not far behind.

However, it takes time to get quality content, especially if you didn't have a game on the drawing board before the system launched. Going to take a awhile to get it together.

It is.

You're right that there aren't many AAA titles from 3rd parties that are either exclusives or released on the same day as other platforms.

There is likely to be a quite a few of them revealed between now and E3 however, even if these kinds of titles are unlikley to be what the Switch will be making its money or building it legacy on.

They didn't think it would be a success. The lineup during that first year speaks volumes, barely any third party support apart from ports.

Was the first year of the PS4 or Xbox One much better?

All those games you mentioned would need a better CPU for a quick port. Otherwise a lot of work would be necessary.

Rocket league runs on Switch with a very low resolution and we know it's not a CPU heavy game as a Sport game. Doom framerate has being cut in half and resolution is also very very low on switch. 

A game like Far Cry 5 would need to be totally redone for switch. Too expensive. It's much easier to port a X360 game (what they did with Skyrim and last FIFA). Unfortunately they don't do recent AAA games on PS360.

I would say it looks good enough to be a next gen exclusive title and the director of the game seems to implicate a hardware limitation with the Switch as well to run the game, the game has some of the more geometrically complex scenes of this generation ... (MHW definitely doesn't look like a game that's possible on last generation systems) 

A game doesn't need to be 1080p/60fps on PS4 like we see with a bunch of last gen ports or cross gen games on PS4 but I've yet to see the Switch being able to run a ground built up current gen exclusive 30fps game for HD twins ... 

I can imagine that most 30fps games that are built solely for current gen leave the Switch out of the picture ... 

Doesn't look all that complex compared to the more high end titles from Western devs. Rise of the Tomb Raider looks more complex in most ways.

You could port just about any PS4/Xbone game to the PS3/360 if you were willing make enough cuts; remove and simplify effects, reduce asset quality and density, trim rendering resolution, etc. And Switch is a fair bit more capable than PS3/360. Doing a RorTR type down-port of something like Witcher 3 might not be worth it, but plenty of other titles could justify and recoup their porting costs and turn a nice profit. Whether third parties are smart enough to seize these opportunities is another matter.

After taking a closer look at RotTR, the only thing that's more complex about it compared to MHW are it's textures and post-processing pipeline. Everything else from the game logic, physics, level of detail and maybe even the material pipeline is done better in MHW. RotTR has roots arguably closer to last gen technology than MHW does and it shows since the developers behind RotTR had to make more compromises in order to get the game to run on a last gen system than the developers behind MHW. RotTR was highly overrated from a technical perspective and I couldn't understand why my PC fellows praise it so much when I think it's just average ... 

A port stops being a "port" once you change some of the fundamentals like the content or the mechanics ... 

A Switch "port" of 30fps current generation title will need more than just graphical tweaks than just asset quality and resolution ... 

MHW doesn't have much going on in terms of physics and game logic, in that regard it's not far beyond some PS3/360 games.

And you wouldn't need to change the mechanics or content of 30fps games like Dark Souls 3 or Monster Hunter World to get them on Switch; nothing about the actual gameplay of these titles is beyond what Switch has already shown it can handle.