Keeping in mind, this is speculation on both of our parts since neither of us have the market data available to companies like Larian or Bioware.
Mmmm. I attribute the high numbers to a) the Baldur's Gate name
Most gamers today likely do not even know what Baldur's Gate even is, in terms of name recognition, I would say it ranks fairly low. Still one of my all time favorite games, but if I asked someone on the street what is Baldur's Gate they would probably think its a geographic location vs if I asked them what is Diablo, there is a chance they would correctly state that it is a game. Even if you polled people exclusively who play games, you probably won't see much in the way of name recognition. Baldur's Gate does have a cult following, but its important to recognize it is just that... A cult following. It isn't a high level of brand awareness.
The market for D&D products is at most (taking the most positive number I have seen posted anywhere) 30m, more realistically 10m and pessimistically something like 2m for currently released products. Of this market, there will be some overlap (but not 100% overlap) with people who buy video games as opposed to buying tabletop products. In contrast, the market for video games is something like 300m+ people who are actively purchasing games. Video games is one of the top 5 largest markets for entertainment in the world and its fast climbing that list. Skyrim sold over 30m copies. There is a larger market for Elder Scrolls Games than for D&D as a whole.
Probably does come into play to some extent. I wouldn't say that its as known as EA or Bethesda, but it is definitely more well known than Tactical Adventures.
I would attribute the sales to the following.
1. Marketing. Larian has heavily marketed the game pretty much everywhere. It was the front page of steam for a while. It was a demo game in new apple tech demonstrations. Its been at large conventions. You get the idea.
2. Larian's Reputation.
3. D&D Product.
This misses the point however. The point is that, as you have correctly stated.
Tactical Adventures is unknown company making a low budget, combat-oriented, SRD game with next to zero hype around it.
As opposed to Larian, which is not an unknown company and is not making a low budget game and there is hype around it. In other words, the audience for the game is almost entirely different to the audience for Solasta. Comparing the games is an apples and bananas comparison, they are not trying to appeal to the same audience. If we took the maximum steamspy number for Tactical Adventures (50,000) and the minimum for BG 3(1,000,000) and say that 100% of the people who play solasta care about mechanics and dislike the changes made in BG 3, they would still only account for 5% of the players in BG 3. In other words, a tiny minority. Not the same market at all.
Were they to get the Baldurs Gate 4 contract I think the numbers would be comparable.
Unlikely. If you look at the sales figures for Western RPGs, most Western RPGs do not break past the 1m-2m sales figures range. Pretty much the only exceptions to this are the games made by 4 companies. Bethesda, Bioware, CDPR and now Larian with D:OS 2. If you look at the games in particular which sold that much, you will notice none of them are particularly mechanically complex. Baldur's Gate 1 and 2 are also not the only D&D games produced, there are others and we can look at how many copies they sold. Neverwinter Nights, which was also made by Bioware (so it had company recognition as it was by the company which made BG), sold ~2.6m copies and was about as Faithful to the edition as the BG games were. NWN 2 was unfortunately delisted on steam at some point and so its sales figures are not reliable, but given the history of obsidian, its safe to assume it did not sell more than NWN 1. Pathfinder: Kingmaker is also in the 1m-2m sales figures range. All of these games were games which had brand recognition for some existing IP and none of them surpassed the sales of D:OS 2.