Why do we use "a" in "take a taxi" and use "the" in "take the bus"? What's the difference?
Expanding on the comments a bit...
If you want to go somewhere by bus, you have to take the correct one. If you take the wrong bus, you will end up somewhere else. So you need to take the bus that goes to your destination.
A taxi will go wherever you want it to. In that sense, they are all interchangeable. So you can just take a taxi, and it will get you to where you were going.