It has to do with the math, the commissions. An extra $10,000 in the selling price is a LOT to you, but to the agent who gets a commission of 3%, it's only an extra $300. Most won't work that hard to get that extra $300. They'd rather sell and move on to the other listing.
Fast sales are key because most of them have put their own money on the line to advertise your home. If your home doesn't sell or sells slowly, that's money out of their pocket or lost. Since agents have their own money on the line (usually), they go one of two ways:
1. They invest as LITTLE as possible in advertising your home. That way they keep their risk low. And of course this sucks for you. These are Realtors that you want to avoid.
2. The GOOD ones know that with proper investment in advertising and with their own skills and reading of the market, they will sell the home and recoup their costs. And this is good for you. These ones generally put the time in to learn the systems, tools, and various forms of internet marketing and data gathering because, hey, they have a lot of their own money on the line.