The reason is the simple fact that Twitter can only store so much data for so long, and even if they do keep the data, it gets so voluminous that real-time searches are next to impossible.
Conquent has created a tool to help out with this -- we can grab and store, ahem, "tweets" for future posterity. Most recently we did this for @FrankAdman's #Twittertini Christmas party, set in the mid 1960s in San Francisco. You can see the archive by visiting tweet.conquent.com/Twittertini
We've used the same concept to create the dynamic soccer ball at the United Nations Foundation's Malaria awareness website at www.unitedagainstmalria.org:
With this, we also pull Facebook at little videos from 12seconds.tv.
At the end of the day, I feel that if you want to keep track of your words, or what people are saying about you, you gotta grab 'em and hold them tight. Get them someplace where you know you can keep them safe so that if Twitter goes away, you can still reference that strange little idea that came from the stream of consciousness we call the Internet.