If one ever catches on, people will start wanting to use it for more than it was designed for, and it will end up being extended to the same extent USB has
We used to have the "slow port/fast port" dichotomy with USB vs FireWire, but people were too cheap to pay for the fast port and eventually the slow port got fast enough to replace the fast port.
Also, are you going to put two ports on every phone, tablet, etc.? In times when even ultra expensive high margin devices like iPads / iPhones are dropping ports to save money?
We used to have the "slow port/fast port" dichotomy with USB vs FireWire, but people were too cheap to pay for the fast port and eventually the slow port got fast enough to replace the fast port.