[random4369]

There's a few reasons, but they're all becoming irrelevant. I'm going to focus on Sony as the exemplar mirrorless player because I'm most familiar with them.

Mostly, it's because mirrorless lenses were garbage compared to Canon/Nikon until recently. This is the biggest reason. Sony in in particular has been very aggressive in addressing this issue by releasing native lenses which compete with Canon/Nikon in terms of price and performance at the high end. This wasn't an innovation problem, this was just a matter of Sony catching up to the leading players. When they did this, 3rd party players like Tamron and Sigma responded by producing more mirrorless lenses as the market size grew.

Next was brand value and inertia. Despite those, Sony has gone from a prosumer toy to a serious consideration in the pro space over just a few years. I don't think this shift could have happened much sooner.

Finally, and this only applies to Canon, it's the firmware. Through a series of happy accidents, Cannon cameras ended up with a completely open sourced firmware package called Magic Lantern which unlocks functionality not found in any other cameras - functionality that enables some types of photography that simply weren't accessible without 6 figure budgets before. This hasn't happened in the mirrorless space, so there's a segment of Canon users who are ready to make the jump to Sony but are held back by its comparatively limited stock firmware.

Also, there is one limiting factor on mirrorless cameras that takes away their main advantage at the high end. Professional grade lenses are big and heavy, and mirrorless cameras do nothing to remedy that. It's just a physical limitation of optical systems, the performance of the lens is ultimately dictated by the diameter of the elements. So if you're a pro photographer with his 85mm/f1.4 or 70-200/f2.8 lens, the size of that lens isn't going to change, and it completely dominates the size of the camera body. This takes away one of the main advantages of mirrorless technology in the pro space.

[st26]

Professional grade lenses are big and heavy, and mirrorless cameras do nothing to remedy that.

In fact, I added a battery grip to my APS-C body for long days shooting with my 400/f4 to add grip surface & balance the weight. I was getting carpel tunnel before I added the grip.

[CydeWeys]

Not even garbage necessarily, just a limited choice in lenses. The mirrorless lenses are being made by the same high quality established companies as the DSLR lenses, they just have fewer choices. To take MFT as an example, which is the biggest mirrorless lens mount by far, it has lenses by several different manufacturers but still doesn't have the same variety available of either the Canon or Nikon full-frame DSLR mounts. Here's the MFT lenses: http://www.four-thirds.org/en/microft/lens_chart.html