The market for notebooks demanded slim and light notebooks. The mobile processors like the MMC-1, MMC-2 and the Mini-Cartridge presented so far all required plenty of space. Also the height of the mating connector and the module are about 10mm. When you want to build a notebook with a total height including case, display and motherboard of less then 3 cm you have to start looking for a different CPU connector solution.
With the on-die level two cache there is no need for an additional PCB like the MMC-1 or MMC-2.
BGA-1 CPU top view
BGA-1 CPU bottom view
When the CPU is mounted, the height of the BGA-1 processor is only between ~ 2.5mm, about a quarter of a mounted MMC-1 or MMC-2 module height.