What technical reason would cause photographes to prefer mirror slap over the near-silent operation of a mirrorless?
My DSLR's are very well developed for very fast focus in very low light ( Nikon D700, D800). The optical viewfinder has zero lag, and works under all light conditions.
My mirrorless cameras (Nikon V1, Sony A6000) have electronic viewfinders (EVF) that have a lot of lag in dark conditions, making the camera almost useless at times. The Nikon V1 has the best on-sensor AF I've seen, and is the closest to a DSLR of any mirrorless camera. But it and the Sony A6000 simply struggle under low light conditions.
EVF's are fantastic for video, and (with "focus peaking") great for use with manual focus lenses. DSLR optical viewfinders are blind when shooting video. But EVFs are not up to the level of a DSLR for stills. That's why I shoot both.