Elgg themes and followup developments

I notice that when it comes to themes that elgg is a bit behind. Now why is it that themes are so limited and are not updated by those who had previously created them? Is it that elgg change their coding too often for developers/designers to keep up with?

I search google for themes for elgg and most of them are for 1.7 and majority are not updated. Why dont elgg have a theming market with active development?