Trust me, we've asked ourselves the same question and really got a case of the green-eyed monster when we saw all the files ahead of us going through. Well, it seems like some of it comes down to bad luck as well
but we were lucky insofar as we were not requested for additional documents and we tried to be as thorough as possible when we applied.
However, my husband is a non-visa exempt file, and the delay in getting police/security/background checks from his country of birth might have had something to do with the processing time (there was a corrupt/violent election going on, with government offices closed for a month in between).
Additionally, I feel that I can safely attribute some of it to the strikes at the London VO from ~July to September, as well as the fact that we applied in June (which obviously meant that December holidays got in the way -- which, mind you, actually does slow down a lot of processing). If I'm not mistaken, June is also the period when CIC fills in new hires, and there is some employee turnover, which leads to less efficiency. June really does happen to be the worst time to apply!
Overall, I believe that though we're a rather straightforward case (young couple, no kids, no military past, etc), the additional 2-3 months of processing time can possibly be attributed to poor communication between countries, the strike, and the extra holidays in between. All of this was made much harder on us because I've been here (and haven't been able to visit him because I'm a student with financial constraints). Hope that helps!