Hi rideflat43, I think because of all the freecoasters that are also used by street riders at least the freecoaster market is 95 % 14 mm. Let's say like this? What hub can be bought with a 3/8 " axle at all? I could not even name five.
And I see most frames have 14 mm dropouts as well, even the flatland frames. There are some with 3/8 " dropouts as the FEC, but not so many more.
I wouldn't say that you need it for flatland (although I like it much more for more than 20 years now) in every case, but the market has made a decision. Don't you think?