When CAMM was announced, they (Dell) mentioned that one of the reasons for soldered RAM was due to electrical tolerances not being met anymore with regular DIMMs at the speeds they were reaching. CAMM was designed to avoid this, and ensures that each trace has the same length so there aren’t timing issues.

I’m no expert but it sounds plausible to me. From a manufacturing perspective, it makes sense that they’d want modular RAM so they can configure them at point of sale instead of having to manufacture multiple motherboards with only RAM sizes being different.