Because Japanese 3rd parties are typically slow to transition to a new system.
Yes. In the lead up to the Switch unveil, I tried mentioning that it took 9 months for Japan to start supporting the 3DS, and it was closer to 12-18 months for the Vita and PS4 depending on what you count as real support.
Given the state of the industry, I don't think it's unrealistic that it takes closer to the long end of the Vita/PS4 support timeline. Japanese companies are overwhelmingly propped up on successful mobile and/or Western businesses at this point, so there's not much of an incentive for them to hurry and support the Switch.
Square Enix: On track to be a $1 billion a year mobile company, had FF15 and Nier notably overperform overseas. Actually perhaps the Switch's biggest supporter despite this, but clearly one of Japan's biggest risk takers.
Bandai Namco: May actually already be a $1 billion a year mobile company. Mostly sells conservative consoles and portable licensed games where platform doesn't matter, while their biggest dedicated game hits are 80-90%+ Western focused titles like Naruto, Tekken, Dragon Ball, and the Souls series. They'll show up on Switch when it's convenient and the primary audience for their cheapy licensed games is there. Has several announced already, but they're coming in closer to the 9-12 month mark.
Koei Tecmo: Working heavily on mobile success with partners. On console, their internal games are targeting overseas audiences (either in the West or Asia), and their Switch games are straight up made with Nintendo. Has expressed interest in larger scale Switch support as their technology for the platform matures.
Sega: I'm not sure what people wanted that they're not providing. Both Sonic games are there, and Atlus announced a new SMT title. However, Atlus is unlikely to launch a new major game for several years, so that inherently fits with slow support. Yakuza isn't there, but by Nagoshi's own words, it targets older Japanese men, who are more of a PlayStation audience. It's not clear Sega has anything else in development. Sega is also a $500+ million a year mobile company with a strong Western PC business to boot, so no pressure there.
Level-5: They're releasing 3DS and mobile games because their primary audiences are children and casual gamers. Even Nintendo is still focusing on those platforms for those audiences. The Switch is expensive and has a limited audience right now, so that makes sense. Level-5 barely talks about Ni No Kuni because it's really not part of their business strategy, and rather part of Namco's big (by their standards) Western-targeted game strategy. That said, they seem to have an Inazuma Eleven game in the works for Switch.
And then there's Capcom. As far as I can tell, most of the panic revolves around one game from this company. Sure, they're not a huge success, but they've had four years in a row of operating margin growth, and seem to have refocused on their historically successful Western console business. That Monster Hunter followed that trend really isn't the end of the world for anyone involved, but people have enshrined that game as such a crown jewel in their mind that both sides go completely ballistic over where the franchise shows up.
And, as another future prognostication, the Switch will never even get close to the volume of games the 3DS had, not because third parties will never care, but because it takes endlessly more resources to make a Switch game than it took to make a 3DS game, and no one is going to increase their Japanese game investment by 4-8 times to keep up that volume.