SODIMMs are huge compared to a BGA memory package which is a problem if your goal is to minimize your board size (e.g. I don't think there's a reasonable way to fit it into a Raspberry Pi form factor without something weird and expensive like a mezzanine connector). Routing the signals is also somewhat more annoying because they all come out of one edge of the connector compared to a BGA package which has them fan out in every direction, giving more space for length matching traces, etc. You'll likely need additional PCB layers compared to a BGA chip.