Lizardfolk are my favorite race in DnD and I'd love to see them added to BG3. That said, the reason they weren't is fairly simple. Larian only had so many resources to put into the game, and the more they added the longer it'd be until launch when they could finally profit off their hard work.
Datamining actually found race tags for a ton of races, Lizardfolk included. So it seems there were early plans for a huge playable roster, but it all likely got sacked because even Larian needs to release the game eventually or they'll go bankrupt. So they decided to stick to the PHB plus a couple of story relevant additions like Githyanki and Drow.
