I suppose hundreds of people have studied and written about this, but here is my (largely uninformed) opinion, anyway.
Representative democracy means that we elect people to represent us in the various governing bodies that make laws, define policy, and enforce laws. Elections are expensive and only occur at fixed times. We only get one chance every few years to elect a particular representative to a particular office. When political figures run for office, they typically have opinions on a number of issues. After they are elected to office, these issues may or may not actually come up in legislation in front of that particular legislative body.
People running for office on a national level, let’s say, the House of Representatives, put forth opinions on lots of issues. There are only two major parties and they try to distinguish the candidates, based on their approaches to the major issues. But, we are voting for the candidates collective views, not any single view in particular. In essence, we are voting for the general idea that the person represents, not their view on any particular issue.
This is why representative democracy isn’t. The candidates aren’t elected based on their views on any particular issue. if, for instance, I hate everything a candidate stands for, except for one issue that is extremely important to me, I may actually vote FOR that candidate, on that issue alone. The candidate has no way of knowing what that particular vote stood for, unless the voter contacts the candidate directly and tells him/her that is what the vote was for. And, most people don’t contact their representatives. Not only that, but by the time the next election comes around, the representatives views/votes on that particular issue may be obscured by the importance of the newer issues.
With the general availability of computers and the rise of petitioning, there is a chance that representatives are becoming more responsive to voters. But it is still hard to tell if a representative is truly representing his/her constituency.