It's unclear why.
Grand Theft Auto IV

A few hours ago this evening, users begin to notice that Grand Theft Auto IV was no longer able to be purchased through Steam. This was also echoed in a package change for Grand Theft Auto IV that was caught by SteamDB. This impacts both the standalone version of Grand Theft Auto IV and the Complete Edition that includes the game's two expansions.

As of right now, Rockstar has not made any public statement about why the game was pulled from sale on Steam. In 2018, the game was updated to remove a fairly sizable number of songs after the licensing rights for the songs had expired. Though some users suspect a similar situation has happened now, the 2018 situation never resulted in the game being pulled from sale for any length of time.

Grand Theft Auto IV also still makes use of Games for Windows Live for its online component. It is possible that it was pulled in order to be re-released with that component removed in favor of something like Steamworks or their own first-party solution. Again, Rockstar has not said anything, so all we have are assumptions to go on here.

It's also a very real possibility that the game is forever pulled from Steam. Maybe it will end up going to the Rockstar Launcher, maybe the Epic Games Launcher, or maybe even nowhere. Until Rockstar says or does something, it's just wild speculation. Right now, Grand Theft Auto IV isn't even available on Rockstar's own launcher anyway.

If you already owned the game through Steam, you can obviously still download and play it through Steam. To add, the standalone version of The Lost and Damned and The Ballad of Gay Tony can still be purchased through Steam.