I mean, this is a pretty straightforward issue. porting takes money and labor, and Nintendo's labor is limited. You can suggest outsourcing to 3rd parties, but a bad 3rd party port is worse than no port. Some companies don't care (see: Sega and honestly most Japanese companies until Capcom c. 2016 or so), but Nintendo has generally always had a high bar of quality with the titles they publish, and especially IP's they own.

It's not even a Japanese issue. Every company has this issue, but Nintendo is the oldest game developer. So they get the most flack