Why?
Simply put, there are way too many Simpsons' references being used in modern day conversation. But why are they used so often?
First off, there is a large body of work to reference. The Simpsons span at least fourteen years, approximately 300 episodes. In that time, they have covered nearly every topic in the known universe, and thus no matter what topic you are discussing there is a relevant Simpsons reference to spice up the conversation.
Then, there's the fact that "everybody" watches the Simpsons. GangstaFeelsGood states in his writeup about The Simpsons:
A 1999 survey conducted by Roper Starch Worldwide found that 91 percent of American children between the ages of 10 and 17 could identify members of the Simpson family. 84 percent of adults could identify them.
Now that's a very high percentage, greater than Star Trek in its heyday - the early 1970's. With that sort of viewership, a Simpsons quote will be recognized by the majority, and everyone will think it is funny as they recall the particular episode and the lead-up to the reference. As well, the Simpsons are always on TV in the form of re-runs, so we see the episodes over and over, every time learning new lines to drop in the future.
Lastly, there's the peculiar ability of quotes from the Simpsons to stick in your brain and never let go. Who can forget such classic lines like "It tastes like burning," "Duffman! Can't breathe!" or "I bent my wookiee." The bizarre power of Simpsons writers to craft one-liners that embed to deeply into your brain is something to behold.
Now, if Fox Television had some sort of device that could scan the brains of everyone in the world and detect Simpsons quotes, and then could bill the referers for copyright infringement or something, then they would quickly amass more money than Microsoft. There's billions to be made from the word "d'oh", alone!