The simple answer is hosting. Most of us pay the bare minimum for website hosting, while sites like BeatStars and Airbit pay hundreds to thousands of dollars per month for their web host.
There are many other differences beyond hosting.
The Music Maker theme allows you to change code, add plugins, upload images, customize the layout, and more. Those other sites are very limited with fewer features than the Music Maker theme.
They also have control over what server resources you take up.
With WordPress you can change and add so many different features to your site, that it can be more than your server can handle at times.
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