Because classic rims brakes work just fine, and there was no need for a new standard.
IMO direct mount was just a half-assed attempt to introduce a new standard to make existing frames and forks obsolete, but they then found a better way that also forces customers to buy new wheels: disc brakes =0
I actually agree with this.
I use to race and train all over the mountains of S Calif and never felt like my brakes could be better. In fact all braking is dependent upon the tire's adhesion to the surface it's on. If I can hit the front rim brakes hard enough to cause the front tire to lock up how much faster can a DM or a disk brake stop than doing that? THEY CAN'T! And besides rim brakes are actually better in mountains because the entire rim, which is a disk onto itself, cools better due to the surface area, thus there is less brake fade. Obviously with rim brakes some companies like Shimano makes what I consider to be poor pads, so I only use KoolStop Salmon pads. And then consider to that for years riders have raced down some of the steepest mountains in the Europe reaching speeds of 65 to 70 mph and there isn't a big area of crosses for all the riders that couldn't stop due to crappy rim brakes!
Just month ago a friend bought a road bike with disk brakes so we decided to test the stopping ability of his vs mine with the Koolstops, he felt for sure his were going to be better. So we did 3 test at 20 mph and 3 at 25 mph, in all
6 tests we were equal to within a foot, sometimes I stopped quicker and sometimes he did (which I'm sure is due to reaction timing) but never was either of us more than a foot difference. While it wasn't raining when we did the test, perhaps in the rain he might have a slight advantage, but not sure if that's true since the KoolStops have an instant response even when wet compared to Shimano which were slow to react when wet; I am also pretty sure from my experience with Shimano pads that had the test been done with those pads on he probably would have stopped quicker in all the tests. So yes good rim brake pads like the KoolStop Salmon makes a big difference dry or wet.
Disk brakes are mandatory for CF wheels however, which is why disk brakes are being forced on us. For those of us on regular AL wheels the need simply isn't there. CF rims do not dissipate the heat like AL rims do, and thus they can delaminate from the heat and the rim could fail if it got to hot, not to mentioned increased braking distances due to extreme fade.
Of course I hear the argument about rim brakes wear out the rims, this is true but only after about 35,000 to 40,000 miles unless you don't clean your rims and pads then you could wear them out faster. But the cost of replacing the rims is actually close to the same as replacing all the pads and rotors over that same number of miles, and rim pads are cheap and the KoolStops last a very long time.