RTwP given that Baldur's Gate and its sequel sold more than any other individual turn based western RPG released before and for many years after. D:OS is the first to match/exceed it. This particular subgenre of RPG was abandoned for close to a decade because publishers didn't see big bucks in it.
The computer RPG genre was all but dead from the mid-90's onwards (barring the brief late 90's/early 2000's revival Baldur's Gate was a part of). Attributing Baldur's Gate success purely to it being RTwP is foolish when there's barely a sample size of relevant games to compare it with.
But why are we even speculating about whether Bioware's success had to do with RTwP, when Bioware abandoned party-based RTwP gameplay after Baldur's Gate, only revisiting it briefly with Dragon Age: Origins. Neverwinter Nights is technically RTwP, but since you only ever control a single character, it being RTwP is almost entirely pointless. Kotor too is technically RTwP, but the consolized interface restricts how much control you have over your party members, so you end up controlling one character while party AI controls the rest.