To the whole nostalgia thing. BG3 exists because people have nostalgia for BG1 and 2, as well as there being many people, such as myself, finding the games very enjoyable in the modern age. Generally, when you make a sequel in a series, you want it to feel somewhat like the previous game, feel like a part of the series. And currently BG3 doesn't have that yet because of many reasons. The first being that 5e is fundamentally different from 2e. The second being that larian has a much different feel than age old bioware or even beamdog. The story as of yet doesn't feel connected enough but we have been assured that it will eventually. And the last reason is a ton of small tonal differences that add up. The party size is one of those differences, and while small is also significant enough to warrant 23 pages of discussion. Ultimately, moving to 6 would in fact allow BG3 to feel more as a part of the series. And would allow it to stand out amongst many 3d story driven RPGS, as Bioware's Dragon Age has only party sizes of 4, and DOS2 only had a party size of 4. In fact, a party size of four or smaller is a product of limitations after going into 3d, because games and I genuinely believe we have moved past those limitations. Dragon Age Origins would genuinely run worse if you had more companions, to the point where that final battle is very glitchy in my experience. Similar for ME if you mod it in, KOTOR as well if you increase the size (at least in experience). So ultimately the conclusion of my rambling is that a party size of four is in fact a relic of limitations that we have clearly moved past, and that we can easily achieve the numbers the 2d games had, and that something being old or nostalgic doesn't devalue it in any way.
But comparisons to the DA and D:OS games specifically are inappropriate, because many (myself included) have pointed out that those games have a fundamental difference with D&D, namely that they are classless (or at least having only weak pseudo-classes, and also just a few such classes at that), whereas D&D is the quintessential class-based cRPG system out there. As such, with a great many, largely different classes that make up fundamental gameplay, not to mention such things as subclasses, archetypes and multiclassing, there is a rationale for party size 4 being acceptable for DA and D:OS while party size 6 being a better fit for D&D.