It also started with Shimano wanting to push Centerlock hubs and rotors down our throats. Since one of the arguments for said system was to reuse the cassette tool to work on them, they needed something smaller than 20mm in the front.
They eventually had to get the 20mm version out or the Saint, Hone and Zee groupsets wouldn't have happened. Their tired motto of 6-bolt rotors coming loose was never met in reality, and in contrast most of their lower gamma CL rotors developed play either at the rivets or at the hub interface.
But then it was too late, they had unleashed that abomination on us. And then they pushed out 20mm boost, just to make the wagon wheels 2% stiffer, instead of relying on wider flanges and a slightly smaller rim diameter.
Trek was their main partner in all of that nonsense. Just as they paired with SRAM to bring out metric shocks.