Because none of the logical options work, apparently.
As you can see in the image above, there are two gear selector buttons built into the steering wheel. One puts the car into D (drive) and the other somehow selects Neutral, Park, or Reverse (no, it doesn't automatically find the local NPR station, despite what the letters on the patent drawing suggest). Just like any other shifting mechanism, the driver needs to press on the brake pedal before the car will change gears.
Why would anyone want to change gears with buttons on a steering wheel? Well, Ford's reasoning is that a system like this takes up less room and "the location of user shift selection on a center console or instrument panel may detract from aesthetics and limit styling flexibility."
But here's the part that would make this sort of shifter more confusing than what we have now:
The progression of transmission mode shifting corresponding to the second switch occurs in a terminal sequence. That is, a first user input of the second switch prompts a shift into neutral mode. A subsequent second user input prompts a switch into park mode. A third user input to the second switch causes a shift into reverse mode. Additional user inputs at the second switch may cause no further transmission shifting actions. User input to the first switch would then be required to cause a shift out of reverse mode. The N-P-R terminal sequence progression of input to the second switch may help reduce inadvertent overshooting of the desired transmission mode by providing only three steps having a stop point at the end. A user would affirmatively provide input at the first switch in order to reset the sequence of modes activated by the second switch.
In other words, you cycle through neutral, park, and reverse with the left button, but when you get to reverse, you can't get back to park without first putting the car into drive. Seems convoluted, no? That's just one idea Ford is offering in this patent, with another being that you could endlessly cycle through N-P-R-N-P... by repeatedly hitting the left button. Of maybe you could just hold the NPR button for a longer time to activate reverse. Basically, Ford hasn't figured out the best way to implement this, and seems to be covering its options in case it ever wants to put PRNDL controls onto your steering wheel.
Ford filed the patent back in November 2015, and given the problems with non-traditional gear selectors – see the 1.1 million vehicles with console-mounted shifters that Fiat Chrysler Automobiles had to recall – we won't be surprised if this idea never leaves the confines of the U.S. Patent Office. This is an idea someone at Ford thought was good enough to write out, though, so keep an eye out.
Source: U.S. Patent & Trademark Office