The simple answer is: the cost of the raw LEDs. Even when you're buying large quantities of LEDs (we typically deal with quantities of 50,000+ LEDs per colour) there's a significant price delta between red and pretty much anything else. With 512 LEDs on a single board, that's most of the cost right there.
We'd love to be able to offer other colours at the same price, but it's just not viable. If you tried to make one of these boards yourself it would cost hundreds of dollars just in raw parts: the only reason we can do it for the price we do is because we buy LEDs in such large volumes.
The Dot Matrix Display (DMD) is a 32x16 array of high-brightness LEDs for visually striking effects. [Product Page]