From what I know, because it sounds good.
A scale is basically a collection of notes which sound "good" together. It's just a matter of convention regarding which notes they chose for that scale. In some other scale there may be tones which are differently arranged...
Think of it this way, they first discovered that sounds repeat themselves after certain intervals (octaves) and also that certain notes sound "good" together... One of these collections of notes which is used in a lot of music happens to be arranged that way. And we have taken to naming other note arrangements using those conventions. i.e. They compare other scales to that scale's (C major) naming conventions.
If you don't understand, think of it this way: Some music pioneers preferred a certain sequence of notes and NAMED THEM. Now everyone else must compare their sequence of notes to that original scale, because they did it first, and their nomenclature stuck.
I'm not that strong on music theory though... I know this much.
All you need is the passion. Everything else comes after it.