Who says you can't? 29er is the same bead seat diameter as 700C (622mm) and 27.5 is the same bead seat diameter as 650B (584mm), and 700C to 650B conversions are quite common. The issues to be aware of are brakes (assuming a frame designed for 700C/29er wheels, 650B rims will need a longer reach caliper than 700C -- not an issue if the frame uses disk brakes), tire clearance, and possibly ground clearance for crank arms (mostly an issue with fixed gears).