If I were to hazard a guess, they're reusing a mobile board design for this somewhat and at least in mobile applications socketed dimms draw 30-50% more juice than soldered. It could also be the npu or gpu requiring the 5-10% extra memory bandwidth they get from being soldered. I do agree that I don't think it was worth the trade offs from a consumer perspective, but framework seems to generally make good choices so I'm thinking there must've been some outside pressure at play affecting the financials of the project or something.