They did it so there's better support for touch screens. Right click on a surface or other touchscreen capable computer is a long press and takes extra time, where a left click is instant.
Nothing to back this up other than my thoughts on the matter.
That said, I hate it with a passion. So much so I was writing emails and came in search of a solution to this "feature". Input device detection would be super useful for both the developers and us, because then people using a touch screen could have left click and everyone else could have right click like our muscle memory over the last 20+ years of computing has been developed to expect.
Or even a check box in the damn settings