Why would a theme writer put their CSS in one long string? [closed]

The main file is minified, so it will be smaller and faster to load. But, the author provided you with the development file version. style.css and style.dev.css are basically the same files, but the dev version is readable, and the main one minified.

All WordPress default CSS/JS files are provided with normal and minified version too (but .min is used for minified file and file without the .min is normal version).