I too questioned why Rick didn't just have everyone start firing once Negan surprisingly showed himself. My only guess it that either they were further away from that door than it looked, or to get a more accurate shot they would have to show themselves more and perhaps exposing themselves to unknown shooters from the other side. Either way, Rick randomly opening fire didn't get the job done, so whatever the reason, in TWD world I think them blowing Negan away just wasn't as easy as it seemed, no matter how comically easy it looked to just pick him off. Just tell ourselves, "it's a show, and the show needs Negan alive." Overall loved the episode. I think last season and Negan's presence has re-invigorated the show for me, after the disappointing season 6. I look forward to what is to come.