Why? It isn't an indication of any skill or ability to accomplish anything. It is a grind, it is about how often you log in and find time to do whatever this faction wants you to do for rep. It literally tests only the player's patience and absorbs their leisure time. Now its going to absorb less of their leisure time, which they can fill with the things they actually enjoy doing, or they stop playing and paying. It is in that regard a statement of confidence on the part of blizzard that their content is engaging and enjoyable that they are willing to reduce the time sinks that serve only to annoy their playerbase anyway.
Good riddance to every millisecond I would spend on rep that I won't spend thanks to this change, and good on blizzard for recognizing that the rep grinds in MoP were an artificial and needless barrier for people who want to play on alts, and making them less of one.
@Zoma - its implemented the way it is to be simpler on programming and to avoid the nonsense behavior of someone grinding to revered on their main, grinding to revered on an alt at double speed to give the double rep buff to their main for their run to exalted.