I think the reason is that most real numbers are gonna be the result of measurement equipment (for example camera/brightness sensor, or analog audio input). As such , these values are naturally real (analog) values, but they aren't fractions. Think of the vast amount of data in video, image and audio files. They typically make up a largest part of the broadband internet usage. As such, their efficient handling is especially important, or you're gonna mess up a lot of processing power.

Since these (and other) values are typically real values, they are represented by IEEE-754 floats, instead of fractions.