Most western developers abandoned handhelds a long time ago. Mobile is more interesting for them. It's the home console market the most interesting one in that regard.
I would have believed that hardware power is the main excuse for 3rd parties to ignore Nintendo, but the situation of the first two years of the WiiU really proved otherwise. Back in the days when the main competitors were the PS360 and the WiiU, most games could have run on the U as well as on the HD twins.
Most publishers took the chance and released their old games on the new system (Deus Ex, Ninja Gaiden, Batman Arkaham City, Assassins Creed...), just to test the waters. However, part of the problem was exactly that: most people had already played those titles in other systems, or they weren't new enough to compete against recent releases. Granted, some should have sold more (Deus Ex, ACIV...), but some of the best selling titles that got a last gen port (Destiny, GTA V, Far Cry 4, every EA sport, PES, Resident Evil HD...) didn't come to the system, in the monent that Nintendo needed them the most, when they were strugling with HD. Even the games that came (Watch Dogs) arrived late and performed bad as everyone expected. Thus we had the droughts.
Very few studios took the chance and released titles during those droughts, when they could have gotten all the sales. The failure of CoD on those systems was the straw that broke the camel's back, though. If there's a game that sells no matter the time of the release or the state it's in, it's CoD, and it failed two times on the U. I'm sure after that, every studio avoided the system like the plague.
But they shouldn't have! Monster Huntes 3 sold well on the U, but Capcom never tried again, Zombie U almost reached the million, but Ubisoft never made a sequel of that (I'm sure the userbase would have prefered Zombie U2 before Watch Dogs), SE and Namco have a ton of JRPGs on the PS3: Kingdom Hearts titles sell well on Nintendo handhelds, FF remakes/remasters, a Tales of game... , but they didn't port one (or brought them to the west). And someone should have brought sports games, someone! EA tried to boycott the system due to Origin not running on the U, when they could have just kept releasing FIFA, as they did on the Wii! Part of the blame is Nintendo's too. I don't know if a port of GTA V would have been more expensive than the possible profits, but they should have insisted on it, and the same with Bethesda games. Without those titles, the system looked barren during two or three years. And when Nintendo managed to recover, the PS4 and the XBone were out already. The excuse of the weaker hardware was valid after that, but not before.
If something hurt the 3rd pary on the WiiU, was:
-Studios not knowing which titles could make money on the system, thus abandoning it before risking time and money. Some even ignored the machine with some successes achieved, prefering not risking anything instead of pursuing the small but consistent sales some games have, like the Lego games. Also, bad pricing for some games also killed the interest (ehem ME3 more expensive than the MEtrilogy ehem).
-Nintendo not trying to get some key titles to the system (GTA V, Destiny, some Bethesda RPG, if FIFA wasn't avalible, try bringing PES...). Also Nintendo not being able to create content fast enough for the system made it look unappealing, and thus not selling fast enough from the beginning.
-I can't say something about the userbase, because the customer is always right, and if they decide to not purchase, it's their choice. But if they want some genres over others, they could have tried bringing more attention to them, specially now with tons of ways of bringing attention to this matters. Example: OP Rainfall. If that worked with Nintendo, why shouldn't it work with other companies?
Just to be clear, Zombi U never came close to sniffing a million. The game was wildly overtracked by VGChartz, we got actual numbers for it from a NPD leak a while ago I forgot what the actual number was but it was quite mediocre. Ditto for Bayonetta 2.
There's no point in Nintendo really trying to get things like GTA. It just doesn't fit Nintendo's image or brand type and that type of game will never really be a huge seller on a Nintendo console.
Even if it sold 60k instead of 80k, that's a good number for a new IP about something as generic as zombies in a system with a low installbase. The same can be applied for Bayonetta (minus the generic part). The point is Ubisoft could have used the assets from the first game and made a quick sequel, but they didn't. Instead of porting Wath Dogs, game that was sold mostly through hype and arrived to the system when it ran out of it, they could have made Zombie U2, a game that proved somewhat succesful, instead of a game that was released to failed (everyone said WD would fail on the system, and everyone were right).