Steam/PC gamer audience and market share is bigger than the Switch/Nintendo one: PC (Steam represents most of its market) market its over half the size of the console market. The market leader in consoles is Sony with almost half the market, which implies PC has a bigger market size in terms of revenue.
Many of the top PC games like League of Legends or Dungeon Fighter Online are F2P games, so make sure their userbase is also way bigger than the Nintendo one. Most Nintendo games can be emulated in Steam Deck.
I'd say they only made the usual PR campaign of sending some of them to press and influencers. I assume their production numbers aren't so huge, so probably they already sell all the units they can produce by promoting it inside Steam, so no need to promote it elsewhere. Which saves them a huge cost/investment.
I think they also may prefer to keep a low profile during the first year or so, implementing pending stuff, fixing other stuff, tweaking the OS and adding a great catalog with hardore, passionate early adopters only. And then once it's properly matured they can open it to a more mainstream audience.