While getting cancelled before an upcoming season can sometimes appear to be a crappy move by a network, the best case scenario is when it happens early enough so that the creative team can take aim at a predetermined ending that satisfies both longtime fans and viewers that picked the show up along the way.
(And also hopefully in a way that still allows for a potential Season 7 to happen.)
No exact reasons were given for why Major Crimes is coming to a close, and unless some juicy news gets revealed later on, it seems like the cable drama is bowing out with mutual agreement happening all around.