My interpretation is that it's because they make the art for them, not for the audience. In a sense they got lucky that one piece of their work found itself at the right confluence of factors to blow up like it did, because that's not something you can really control or guarantee, unless you play it extremely safe (which is why all the big game/movie studios produce sequel after sequel, or old rockstars will just keep performing their classics).
There's an interview with Jon Blow I heard recently that stuck with me, where he says that he could probably have made lots of money making Braid 2 or The Witness 2, but that it just wasn't interesting to him.
Willingly leaving money on the table, now that's the mark of an artist.
If you're wanting more of The Witness you can find it in childrens puzzles books and the backs of cereal boxes. See also The Looker: https://store.steampowered.com/app/1985690/The_Looker/
- Taiji, which is a very The Witness-inspired 2D exploration/puzzle game https://store.steampowered.com/app/1141580/Taiji/
- Fidel Rescue Dungeon (which I only just learned of and haven't yet played), which is The Witness puzzles, but dressed up as a roguelike https://store.steampowered.com/app/573170/Fidel_Dungeon_Resc...
but sometimes, you get someone who really gets into a small niche in gaming where their other interests are far flung elsewhere, like the ever lovely MOTHER series by Shigasato Itoi