Because the PC market has never been a threat to console gaming. Sure it's a handheld, so naturally there will be comparisons, but outside of emulation (which isn't 100% perfect and certainly not legal if you don't own the games) you aren't getting the 1st party experience you would buy for a Switch anyways. I have both, they both serve different needs. I use my Steam Deck for playing my Steam library when I'm not at the computer, or in bed. I play my Switch to play Switch games.

It'll do reasonably well sales wise, but it's not going to sniff the same level of success as the Switch.

That being said, I hope Nintendo is at least paying attention here. The next Switch has to be at the bare minimum the specs we are getting here in the Deck. That plus DLSS would do wonders.

Room enough for both. Plus, different focus for the people they cater to. Especially if you have young children, the Switch is the safest bet and a great handheld for introducing video games. Steam Deck is a wonderful handheld that allows you to do anything (steam library) but way less security. I don’t see these machine as direct competitors.

2 totally different markets; it's just that simple.

People keep saying it's a different market and I don't get that logic. The Deck is a dedicated handheld first with the option for those who wish to make more use of it. It's within the same price range as the switch, it's dock-able, has similar hardware features, it's not two completely different markets when they're two obviously similar products. Accept it or not, when valve starts to advertise this thing to the mainstream, and they will, it'll be a direct competitor to the Switch.
Now will it outsell the Switch or maybe even take away attention from whatever Ninty does next? It could with enough right marketing, but Nintendo's name alone will probably keep the switch on top. But y'all gotta stop kidding yourselves, you're making up excuses