I think there are some technological limitations behind the 16GB maximum RAM configuration. The highest-density DDR3 DRAM packages I could find that run at the rated speed are 8 gigabits, which means 16 chips are required on the board. There probably just isn't enough room there for 16 more chips.

16-gigabit packages are slowly arriving on the market, but I imagine it'll be a couple more years before they're available at the right speed and sufficient quality/quantity to include in a future MBP model.