Intel's IGP are the most popular video chipsets used by a longshot. Dwarfs Nvidia's market.
I guess it makes sense. But wouldn't a more straightforward explanation be that in the past there were no OpenGL drivers shipped by default with Windows XP? I think you only got them once you downloaded the drivers from the manufacturer site, and probably many people that would potentially use Google Earth would not know anything about this. Nowadays Vista and W7 have OpenGL drivers included by default, so this would not be an issue with these systems.
The reason DirectX gained popularity in the first place for games is because each vendor shipped a different OpenGL stack.
This meant that developers had to troubleshoot and support no less then 3 different OpenGL implementations; each with their own quirks, limitations, and extensions.
With DirectX they only had to support _one_ and that was Microsoft's. It does not really matter which API is better; they do just about the same thing. What matters is that they can always depend on DirectX working the same and being available.