To a degree, IMO, I think it is because both sides have some pretty damaging sound bites on the other guy. If Romney brings up the whole "redistribution of wealth comments" from previous sound bytes and video, then Obama goes the 47% route. Possibly both sides realize that most people are sick and tired of the BS name calling and school yard he said she said crap. I know I am. Talk about your plan and how the other guys won't work.
The bottom line is that each party has their base that is unwavering. Look at this board as an example. With all the cutting and pasting of this link or that link do you think anyone has changed their opinion? I do not. The debates are for the undecided and independents. IMO, both sides simply personally attacking the other guy instead of the plan will push away voters. Hard to like a guy, regardless of policies or plan, if the guy comes across as an arrogant a$$. If you truly are the "right" candidate for the job, let YOUR body of work stand on its own merits. You shouldn't have to win by proving the other guy is an a$$. Again, IMO, this can come across as arrogant and really put voters off.
The recent debate showed reserve by both candidates. The could have gotten in more digs or zingers if you will, but chose to be more civil. I was surprised and even impressed by this. Maybe they are waiting for a later debate to mud sling more. Maybe both will simply leave it to their respective campaigns, proxies and media to do this so as to appear more "noble" one on one.
Goes back to the old adage, don't throw stones if you live in a glass house.