There are going to be a few layers here, but given that it's a Homestuck question, that's far from unusual. Oh, and in case anyone is reading this post and hasn't this post and hasn't caught up with Homestuck yet, or at least reached Act 6 Act 3, spoilers of the largest sort will follow.
Initially, in the B1 universe, multiple copies of the game existed and were distributed(I'm not counting the Alternia universe as "initially", even though it created the B universe set, for reasons I'll get to later). Since Sburb has been compared by Word of Hussie(I don't remember whether it was in the story itself or through Formspring) that the game exists as a sort of cosmic reproductive system, this makes sense. The Genesis Frogs would need to be able to multiply themselves, rather than just existing and creating a new version of themselves every time one dies. There was a map in Act 3 that showed that while there are numerous sessions, there are far more meteors than there are active sessions, but there still are active sessions. The prevalence of the meteors is probably because there are multiple sessions' worth of meteors aimed at the same planet. While The Battlefield is always destined to be destroyed during the Reckoning and the Prospit side is always destined to lose, Skaia still needs to exist to provide the alchemy cradle for the formation of the Genesis Frog, so some, but not all, of the meteors are sent through; ones sessions' worth which would heavily damage a large planet like Earth or Alternia, but not kill it all on its own.
Compare the trolls' world, A2, which only had one session, and the B2 universe, which only had one session, to B1 Earth. The troll world is, compared to the B1 Earth, almost undamaged when shown in the post-session era. Sure, most of its society has been destroyed due to the corrosion of troll cities and infrastructure following Gl'bgolyb psychically killing the planet's population, but a lot of what it is in the Exiles' time is almost the same as it was in the trolls' time. Even the deserts that the trolls' exiles operate out of are still present and widespread in the pre-session days. They can be seen most frequently during Kanaya's segments, as Kanaya lives smack in the middle of said desert. Granted, she lives in an oasis, but she made it herself, so it's not representative.
Both the A2 and B2 planets weren't as badly hurt, and in fact managed to rebuild their societies, albeit through the influence of the Felt(which included an omnipotent First Guardian among their numbers) and a space conqueror respectively, but B1 Earth is irreparably damaged by the onslaught. The only thing we see of it is a continent-wide desert, which means it's likely that the rest of the world is devastated or uninhabitable. So, why is the B1 Earth torched completely while the B2 Earth only has widespread flooding and climate shift? Simple. The B2 Earth existed as an extension of the session played by John, Dave, Jade, and Rose, while the B1 Earth existed to be the starting point for their session and all of the other human sessions.
The Scratch resets only a single session, because as you said, resetting every single session for an entire species because one session isn't working out would be pointless. What if the flawed session was simply unable to win for a reason that was out of anyone's control, as happened in the main story? So, the Scratch creates a new planet with shifted-around versions of the null session's players in an attempt to correct the problem, and retroactively inserts it in the universe set's Genesis Frog.(Remember, when Bec Noir attacked the B universe Genesis Frog, it was revealed that all worlds in the Homestuck continuum containing Earth and its population exist within the Frog's body. All timelines, all doomed timelines, all offshoots, and the like. Since a scratched world contains ts own alpha timeline, it would be an offshoot rather than a doomed timeline, but could still potentially create more doomed timelines itself.)
The B2 universe only had one session, with all of Sburb under wraps, with a closed beta, etcetera, because it only could have one session. It doesn't exist to propagate on its own merits. It's the path of a single ovum, if you'll allow me to compare this to real-world mammalian/amphibian terms here for a moment. The ova cannot create more of themselves after the construction of the ovaries, and thus there is a maximum number of possible successes. Likewise, the scratched universes cannot create more than one new universe, but they can finish where their predecessors failed. Not a perfect comparison, because ova can't go back and try again if they weren't fertilized the first time, but it's the closest I could come up with.
This fits into the setup of the Alternia universe. It is a post-scratch world, designated A2, and was the result of one of the A1 universe's sessions encountering a dangerous and session-fatal glitch. It has not been revealed whether Beforus had more than one session attached to it, but I think it's a likely scenario. And, regardless of what hundreds of fanwriters will tell you, in the Alternia that produced Sollux, Karkat, Kanaya, and all the rest, there was only one session, and it succeeded, creating B1, a new, fertile life form that would eventually produce numerous sessions of its own.
So, to sum it all up, the other B1 sessions do in fact still exist. In fact, they wouldn't have been affected by the Scratch for the same reason the rest of the Medium wasn't affected by it; the Scratch only hits a single Incipisphere, and they're outside its range. We just haven't seen any of them because we're following, in the words of Hussie, "a boy and his friends and the game they play together". Granted, those were circa-Act 4 words and it's a little more complicated now, but they still hold up. The other B1 kids probably either died when the universe's cancer hit the session(likely), performed a scratch of their own(less likely, but distinctly possible), or won(enormously unlikely), so their stories probably wouldn't have even been as fun as the tri-layered, quad-universal adventure Homestuck's been telling us.