The switch is a handheld and the ports it gets are for that reason. It wouldn't have sold enough to get basically anything third party if it was the same device without portability (see BOTW as a system seller when it literally already existed), and it still doesn't really get that many current gen demanding ports.
The fact that there's a worse Xbox you're required to support when the Xbox already lacks some of the asset loading tricks of the PS5 and has less units sold on top of it isn't something developers can just ignore. BG3 really isn't all that demanding for a next gen open world game, and compromising your vision to force it onto a worse console isn't something people want to do.