I was having a discussion with my pal Dave Meyer about this yesterday and we both feel it is simply because there are simply too many moving parts to the ever-evolving puzzle that is MMA. When we have fewer moving pieces (possibilities/techniques) we tend to hone in on one particular aspect more easily and refine and develop it - but with MMA, athletes have to cover so much ground (boxing, kick-boxing, stand-up clinch work, takedowns, BJJ, ground and pound) that most of the time is spent just getting the sparring done (in each of these areas) and participating in the general training for each of these areas (not even considering other things like strength and conditioning) - that there is little or no time left for 'technical development'.
I think this needs to change. Each day - some time should be set aside for very specific techical development. Ie: pick a specific problem/challenge, come up with a solution for it, design a training regime for that solution and work it. Maybe one problem a week - easy - by the end of the year you will have drilled for fifty odd problems. Now I get that this is over-simplifying - but it does need to happen - in some form or other.
Investing a little time in Technical Development - will p[ay huge dividends in the long term.