Two common reasons:
- Shame. Like many artists, some indie gamedevs feel ashamed of their old work, or it brings back bad memories for them (the latter is especially common when indies go out of business).
- Laziness, or at least a desire to avoid needless work. It takes additional work to get a new Steam page up under a single dev’s name if the studio disappears, or to upload the game elsewhere where it can remain available. It’s work to go through all the licences the devs have to see whether they can just make the game freely available (some licences may require additional fees based on installbase and such). It’s easier to just close your store pages and your site and not think about it. Also, if even one person on the team doesn’t want the game to stay up, there’s not much the others can do about it.