It's mostly because of the bad talk that Diamondback's get on the bike shop level. A perfect example of this was at a trip to Ray's, I demo'd the Scratch Air by Trek, and really had fun on it. I then ran into a guy from a LBS in the area that talked with me about what I thought about it, and such. When I mentioned to him that I was thinking about a Diamondback Mission 2, he instantly said, "What? Why? You just tried out a bike that is ten times better then anything Diamondback has to offer. I know. I used to sell them at another shop I used to work at." So, I then probed and asked him "Why?" His response was that the suspension design, even on the Sorties and Missions was too flexy on the rear of the bike. I'm not sure if this guy had only sold them in their first year, or what, but I thought he was off of his rocker when I heard this. Granted, I have heard of the Sortie have a minute amount of rear frame flex, it's nothing that you won't feel in other bikes... and it's probably the wheel frames rather then the swingarm. But anyways, my point is... every bike shop that I've been in, when I objectively ask them what they think of the Diamondback Mission line, to get their opinions, I usually only get negative responses. It's rather funny though. The one shop I did this to shot down the DB Mission, and proceeded to try and convince me to buy a specialized enduro. I then said, "I'll be right back" got the mission out of my car, rolled it in, and asked them if they carried the Jagwire Ripcord cables so I could see how they looked on my bike... the few customers that had overheard the conversation were pretty impressed and the bike tech sneered at me, pointed to the cables, and then walked away. haha.
Anyways, that's my little rant/story. Really, it comes down to the people that do have the Missions, let a riding buddy try it out (if you trust them enough with your bike) and just see how they like it compared to what they have. The people that I have heard of doing this say that their buddies roll back with a huge grin on their face.