They weren't even the first, Ayn and Ayaneo were making handheld gaming computers well before Valve or the other major companies started to get involved.
It's a niche market. Niche markets are always under served.
As great as the steam deck may be, it's only going to appeal to a small percentage of gamers. Generally more tech savvy players who frequently travel some distance with enough down time to justify that purchase. Whether that's commuters or guys who travel for work. Or guys who just want a really expensive controller so they can play from their favorite chair.
device to play the
It actually very likely brought a lot of women to Steam who predominantly played with handhelds because they were indoctrinated to believe PCs are only for "tech savvy men".
December 2021 less than 10 % of all Steam users were women. Between 2022, when the Steam Deck was released, and December 2023 the number shot up to 32 %.
Actually, there are a few different ones. If you look for pocket PCs or even specific for gaming. The mainstream just didn't know about it because they don't have marketing reach.