I think there's a few factors that contributed to it. First off Cree and XM and GE and others are making some very efficient LED's that are now easy to obtain. Even 2 years ago, this was hardly the case.
Secondly, with the massive surge in nano and pico tanks, specifically, very high end ones, LED's are a natural fit. There's a major gap in quality lighting below about 30 - 40 gallon rectangle tank sizes. The T5/T8 lighting doesn't fit very well, and MH on a small tank is a problem in itself. It also doesn't take an extreme amount of LED's to adequately light the tank so they were a really good diving board.
Last, they aren't necessarily more expensive than MH or T5 if you average it out 5 - 10 years. No bulb replacements at all, negligible additional heat.
For large tanks, it's still cost prohibitive for most people but as the efficiency continues to go up and the price continues to go down, it's just a matter of time before LED's are the majority of lights.