Baldur's Gate back in the day sure as heck had impact to game scene, for the better and worse.
For lot's of people with PnP background and experience from Bard's Tales, Gold Box games, Ultima etc. fantasy games, I don't think vast majority hardly knew what was happening in cRPG space and had lost all hope for getting anything for years. Also generation that played PnP / Gold Box etc. games back in the 80's were maturing, had other priorities in life. And then suddenly there is this game where press say it's actually pretty decent, then people say it's actually pretty good. In retrospect it had much to do with about people who liked to try if they could revisit their past PnP / cRPG experiences again, nostalgia. Baldur's Gate was right game at the right time to appeal that niche market.
I also think that big contributor to commercial success was that people even get to know about it. IMO Bioware very consciously took advantage of all brand building TSR/Wizards of the Cost had done, much better than SSI back in the day with their AD&D games. This didn't only helped them to reach people who were interested in cRPG's but gamers in general.
How good cRPG in general Baldur's Gate is in comparison to Fallout? IMO Fallout is in much more sum of more than it's parts than Baldur's Gate, but that's beside the point if you think of target audiences of that era, which somewhat reflect audiences whom cRPG's are targeted even today. (It's important to note though that both games were successful enough to have sequels produced).
Baldur's Gate can be faulted for many things. It feels quite a shallow game when you replay it. That said, experience may vary a much depending how much player is immersed and invested in filling thing with player own imagination. Building the world, including characters populating the world Baldur's Gate had much more defined characters (conveyed to player via party banter, graphical presentation etc.) it was light years ahead of old games, which were much more abstracted and managed to hit some sort of golden ratio between players who liked abstraction, either because they liked to imagine everything, or they approach was (game play) mechanical and those who wanted more story, more elements to create their own fantasy. The latter turned out to be, according the information I'v gathered via Codex and other sources, also the birth of certain phenomena, which ironically has destroyed whatever was left of Bioware. Infamous example is character Imoen, who was supposedly had much more active role in Baldur's Gate but her lines got lost due some mishap in Bioware. Bioware however noticed that many players got really attached to Imoen, because she was so passive. This also brought in folks whom had this fantasy about Imoen and main character romance (they are sisters).
Thinking Baldur's Gate as game on today's, perhaps even past standards, makes it IMO a bit shallow experience. It's appeal IMO rest much more on it's characters and overall, things Gold Box games weren't able to represent due technical limitations of late 80's early 90's tech among other things. But we can ask for example, if campaign overall is as good as it's in Pool of IMO Pools campaign if better. Is it better than in Savage Frontier games? Maybe SF games campaigns didn't exactly had much of flesh to begin with IIRC, so yes.
There is also that scope and focus on Baldur's is entirely different. While I can see many commercially good reasons to make main character a Bhaal Spawn, it's whole different situation for developer and player when plot revolves like that around the main character, which is practically players alter ego. Whatever they thought, IMO for developers I think much better practical example of using a character who has some sort of supposed destiny to begin with, is Final Fantasy VII.
Where Baldur's Gate succeeded was making boring progression from level 1 to 3-6 actually bearable considering how much players need to spend their time to 'explore' the map, which is clicking the mouse. I can't fault Bioware for not having a practical solution for issue we still have today. In PnP I was never told as player, or as DM never told to players to do something silly when I detail the route to their destination (even if there were dice rolls for random encounters where they made sense). In cRPG, there isn't this sort of practically instant travel, but players keep clicking on map, imagining he is exploring, when he is actually just repeating very mundane task. Today we have Shadowrun games, where party uses metro, no need to keep clicking the mouse at all. Of course there was Fallout with large world map where player set destinations, or for more traditional approach, Wasteland 2, where player simply set destinations on map which just is in smaller scale and but has hazards.
But back in the day, I can understand that Bioware wanted to show off and create those sceneries where much of nothing happens, which is actually all right and according to rules of Forgotten Realms settings.
Bioware managed to make a lot of things right. Pulling a campaign with focus on dramatic main character, rule system that is leaves a lot to be desired (AD&D). They also managed to give story and especially characters depth, like never before.
In scope of cRPG AD&D history, say Pools of Radiance and it's sequels, Baldur's Gate managed to tell a different kind of story and be a different kind of game. Baldur's Gate manages to tells a story and be a game of some very few from all of those thousands of NPC's in Forgotten Realms. Few who could had in alternative universe as well left from from Hillsfar whom you never know and never care when if fight for Phlan in Pools of Radiance.
Baldur's Gate succeeded commercially because it was like a drop of water on the desert. It wasn't terrible game, not terrific either and succeeded in marketing and filling certain void that gamers felt had at the time in cRPG space. It left legacies which are for the better and worse. The most important thing however, I think is that Baldur's Gate was IMO succesfull where it tried to best Gold Box games, in a small scale. Now there are much better games out there, but I don't think that would necessarily be so, if Baldur's Gate and Fallout (add Final Fantasy VII) weren't been known and liked (commercially viable) products.