Because despite Bungie claiming that the Atheon fight was always intended to be the way they are making it starting on the next update, nobody believes them that it was a huge coding bug (that somehow miraculously made in through QC) that made Atheon almost always reliably port the three Guardians that are the farthest away.
It seems that some people inside Bungie are very upset that people are developing strategies, posting youtube videos, and lots of people are beating the raid and the hard raid over and over in less time than they had hoped...
so in an effort to try to keep people grinding the raid for longer they are going to make a knee jerk change to the whole encounter that increases the difficulty substantially and further leaves out the more casual players or the players that don't have a regular RAID group of trained assassins.
