Do you have a 2016? There's another thread on here about some changes Ford made only in 2016 to improve the turning radius.
I've never driven an earlier ST but I also have a 16' and don't really have any issues I've noticed with the turning radius.
There's a change in the amount of steering wheel play lock to lock in 2016. There is no official word from Ford on why this change was made. Up to 2015, the steering wheel would turn 1.8 turns lock to lock, now it appears to be about 2.3. Maybe a 1/4 turn in each direction.
SOME have GUESSED that this might be to improve turning radius, but it could also be just a change in the steering ratio. The steering radius of the car is more affected by the size of the wheels and that hasn't been changed. Simply changing the number of turns lock to lock is not going to automatically equate to a tighter turning radius.
I have a 2016 and it turns like a pig. It's great in the bends, but if you have to U-turn on a narrow street.....well, I at least I get good use of my backup camera. If this is "improved" I'd hate to see what it was before.
Since wheels and the depth of the wheel bay also affects turning radius, I highly doubt anything was done that drastic to the car.
Others have guessed that this was to detune the ST so that the RS has the most responsive steering.
No one knows because there is no official documentation. Only the proof that on a 2015 or earlier, the steering wheel is less that 2 turns lock to lock and on the 2016 it's about 2.3.