The reason? It's impossible to convey much information in 140 characters. So when a search hits a tweet you get at most a soundbite, telling you something you probably already knew. When you search you're looking for information you don't have but want.
I have a collection of Google Alerts that report once a day or immediately, via email, telling me about occurrences of my name, products I've made, other topics I'm interested in. These used to be pretty useful until they started including tweets in the body of stuff they search. Now the alerts are mostly useless. So in this case, adding real-time stuff actually subtracts value.
If Twitter wants to make money by inserting ads into search results, and all indications are that they do, they should seriously consider relaxing the 140-character limit, so tweets can carry information worth searching for.
Update: Liz Gannes has the report too.
Josh Young adds: "Yes, real-time results in search suck because the feed is what's important, not the individual tweet."