Nikon made a mirrorless camera series (Nikon 1) that was very capable (the on-sensor AF was great). But they chose a 1" sensor (2.7x crop)

Canon's M-series was similar, they didn't go all in on "pro" features, instead focussed on a consumer market that was shrinking rapidly.

The problem wasn't the technology, the problem was the fear of cannibalizing their own DSLR sales.