Why didn't Cirrus make the SR-22's parachute steerable?
Remember what the purpose of the CAPS/BRS airframe parachute is: It's a last-resort emergency measure to put the aircraft on the ground in a condition that is likely to be survivable for those onboard.

Steerable parachutes add considerable complexity: Control lines would need to be routed into the cabin, and connected in such a way as to not interfere with the chute deployment under any circumstances.
Assuming a suitable design could be devised to accomplish that a steerable parachute (even for a single person) requires quite a bit of training and skill to use. This is not something that can be adequately trained for in the case of the Cirrus airframe chute (because pulling the chute on perfectly good aircraft to teach steering would be cost-prohibitive).

Faced with the risks and complexity of a steerable chute or the relative simplicity and safety of a simple drag chute to decelerate the airframe prior to impact (hoping you land in a suitable 40-by-40 foot square) Cirrus and BRS opted for the latter. The result is a dead-simple "Pull the handle!" system that's proven very successful thus far.