Nintendo has typically been good about getting developers to support their handheld hardware, unlike the more spotty track record of their consoles. That being said, looking back in hindsight, the Nintendo 3DS was an anomaly in that for a Nintendo handheld, it was oddly lackluster in third party support, especially compared to the likes of the DS or GBA.
First, western support took the biggest hit. Back in the DS days, you had everyone from EA, Ubisoft, even Rockstar releasing interesting stuff for the DS. Hell, the DS got versions of every CoD game since Modern Warfare. But with the 3DS, western developers dropped support for it as quickly as they took it up, and they never really picked it back up again either. Japanese developers fared a bit better, with Square Enix and Atlus heavily backing the system. But even that was a bit of a step back from the DS, as many of the more unique games the DS was known for were almost non-existent on the 3DS.
Even indie support was underwhelming. Due to the 3DS' lack of engine support and clunky hardware, many indie games skipped the platform altogether. So, yeah, the 3DS' 3rd party support was disappointing. I love the 3DS, it's up there with Nintendo's best on the first party side. But it's much more of a Nintendo box than some fans like to admit. At best, it's on par with the GameCube and Wii (that is okay, not great, not terrible), and at worst it's a massive step back from its predecessors. The Switch however looks to bring things back to DS and GBA levels of support, especially if more stuff like Octopath an Ninjala keep coming.