[ev0lution]

SA-MP doesn't support the Steam version of SA because it started 10 years ago from the first ever PC release of the game, when Steam didn't exist. The mod also isn't officially supported by Rockstar. These two facts mean it'll never be on the Steam store. (but if all you want is the in-game overlay, just run it as a "Non-Steam Game" through Steam)

To compare, Just Cause 2 was only ever released on Steam, so JC2-MP could be built entirely on the Steam version. That mod was supported significantly (and even promoted) by Square Enix.

It would be awesome if SA-MP used only the Steam version because then we'd have the ultimate ban system - SteamIDs. But this would be completely unfair because I would estimate that 90% of people who bought SA didn't buy it on Steam - they probably bought it from a store or elsewhere. And that's just people who bought the game, I'd also estimate that about 90% of SA-MP players are using a pirated copy, and getting rid of so many players would be catastrophic to the community.

I'll also note that I believe one of the reasons Square Enix supported JC2-MP was because it ran only through Steam and therefore only with legitimate copies.