Since 2009, when the Panasonic G1 was launched as the first mirrorless camera, there have been zillions of discussions about mirrorless vs. DSLR cameras. When reading forums, it's easy to get the impression that mirrorless is taking over the world, but when looking at full frame cameras and what is actually available in the market, the picture looks a bit diferent.
In 2010, the following full frame DSLR bodies were available:
Canon 2, 5D II and 1Ds III
Nikon 3, D700, D3s and D3X
Sony 2, A850 and A900
Within 2015, the following bodies will be available as current models:
Canon 6, 6D, 5D IV (rumoured), 5Ds, 5Ds R, 1D X, 1D C (or replacement)
Nikon 6, D610, D750, Df, D810, D4s, D4X (rumoured)
Sony 1, A99 (hybrid)
When it comes to mirrorless, the figures are much simpler. There was none and now there are three. My guess is that there will be three at the end of the year as well, unless Nikon comes up with a surprise.
The reason for this, I believe, is that while the camera manufacturers in general are struggling to keep up volume, the high-end DSLR market is still profitable. Technological progress isn't reserved for mirrorless cameras, and when cameras that come in a package that is known to most users continue to improve, few people see any reason to switch.
Although the Sony A7 models are technologically great, the fact that Sony is the only player in the full frame mirrorless market might even be a disadvantage for Sony. Sony have gone down obscure paths alone on several occasions, and many have been dead ends. People know that and hold back. The management at Canon and Nikon also know that, and one of the reasons for holding back on their mirrorless efforts may simply be that it would confirm to the buying public that Sony was right after all.
As it is now, the can play the game of FUD, and if they have the technology ready (which I don't doubt that they have), they can launch their mirrorless supercameras whenever they feel that the time is right, letting Sony carry the cost of pioneering in the meantime. That is a disadvantage to mirrorless users, since little competition often means slow progress.