And that's all the explanation needed.
Maybe if you only care about factual continuity, but that's one of the least important factors in the decision. Coulson's death in
The Avengers was an immensely powerful moment
emotionally, not just as some cold, shallow factoid. And so bringing him back in a later movie without giving his return some emotional meaning and weight would be wrong. It would be as if in
Return of the Jedi, Yoda and Obi-Wan had told Luke, "Nope, Vader was just kidding you, he's not your dad" and then it were forgotten after that one line. Or, heck, as if they'd opened
Star Trek III with "Captain's log, supplemental. Now that Spock has been successfully resurrected by the Genesis Planet and returned to active duty, we are proceeding to our next assignment." That would obviously be terrible. You simply do not put an emotionally powerful event in one movie and then reverse it with the minimum possible effort in the next. Not unless you're a really, really bad storyteller, which Joss Whedon obviously is not.
The only reason Coulson's resurrection worked dramatically and emotionally in the TV series is that they had the room they needed to make it an ongoing focus of the story and to make his revival a source of problems and personal angst for him, rather than just a handwavey erasure of a powerful moment. The only way it could've made sense to bring him back in the movies is if they could've done something similarly extensive and meaningful, and not only did they not have room, but it'd be kind of redundant since the series has already done it. Sure, the whole discussion is predicated on the fact that many moviegoers won't know about the show, but the show is still part of the larger universe, and you want to make choices that balance the needs of both the individual parts and the larger whole. In both regards, Coulson's life after death is a thread that's better served as part of the TV side of things.