Why the solid 12mm rear axle?
If you mean 12mm thru axles, they have been around for a few years now but in different guises depending on hub and frame spacing standards.

• 150x12mm 
• 142x12mm 
• 135x12mm

With the rise of enduro/am/do it all style mtb
It's about creating harmony between weight, stiffness and ease of use.
and a standard going forward that can adhere to modern frame and hub design trends.

The main advantage of 12mm thru axle is that through axles enable you to get a more secure connection between frame and wheel as the axle goes through the hub and almost always then screwed into the frame. 

The 15mm and 20mm axle standard is geared up more for Forks. and some come as proprietary technology specific to a fork manufacturer. 
Also depending on your riding style and frame dropouts 9mm qr can be weak and flexy thus a 10mm bolt up conversion may provide appropriate strength/rigidity for older frames.