Maybe because it is a different franchise that is known for another system? Maybe because being in the comfort zone (which clearly they opted for) don't expand your audience?
Being in their comfort zone and refining and iterating something that they were familiar with already did expand their audience, with DOS2 outselling 1. Furthermore, WotC contacted Larian during DOS2 prerelease and offered them BG3 because they liked what they saw with that game. Many other studios besides Larian, such as Obsidian and InXile, had pitched to get the BG licensing rights, but WotC wanted the DOS2 model for BG3. Clearly Larian isn’t going to change their approach when that is what their client is requesting.
WoTC: "Hey Larian, we love what you did with DOS2 and we'd love you give our flahship title!"
Larian" "Wow! Great. We're in!"
WoTC: "So what can we expect?"
Larian: "We're gonna roll the dice with something completely new and untested, which may or may not end up being better, but is going to cost a lot more money and time to produce!"
