Because - numeric precision arguments aside, there's an excellent comment explaining that problem in this thread - it's the only unit of measurement that makes

Because - numeric precision arguments aside, there's an excellent comment explaining that problem in this thread - it's the only unit of measurement that makes sense for angles. It provides an easy way to connect the complex exponential with trigonometric functions (and everything you get from that, i.e. Taylor series, nice behavior in diffeqs). You can do the same in degrees as well, but you end up with weird conversion factors with pi in the denominator, a strong indication you should have multiplied by pi to begin with.