I guess the short answer is, we don't know, none of us work for Google, and they're not exactly sharing that info.
If I had to speculate, I would imagine it's one or more of the following: Too busy to get to every country yet; Need to negotiate with foreign governments for access; Problems with cellphone carriers; who knows?
Don't take it personally. I'm in _Canada_ and we don't even have access to paid apps yet! (Too bad I can't drive across the border to buy them! ;-) )