The overarching theme is that the payments industry is much more complicated than it seems to the layman. There are a ton of players that you have to interact with, and they are a different set from country to country.
The payment systems are surprisingly different on the backend in each country. The US is large enough where it makes sense for us to create integrations into all of the different backends to work with all banks/processors. To do it in smaller countries might not make as much sense. We have to tread carefully because we need to ensure that our partners are great. That is something we can do in the US because our partners are a known quantity. Not the case for us abroad.
We also want to be able to provide adequate support. There is even a timezone hurdle. We provide phone support and it would suck to get calls at night.
Another part is just honestly the US market is so large and we still have room to grow here and add awesomeness that it is hard to prioritize going somewhere else. That being said, we are pursuing it because there is OBVIOUSLY a huge need there. We are still in beta so we're still working on the core product. Simultaneously we're working out a solution for the EU (difficult) and Canada (not as difficult).
Other issues include taxes, currency and currency risk, taking payments, etc. There's a lot to think about there. We really want to do it, but to do it right requires a lot of thought, and we just haven't done it yet.
If you have connections to folks that can help, please drop me a line (email in profile)
BTW, if you are interested in geeking out about how processing works (in the US), here's an ebook: http://feefighters.com/ebooks/what-is-credit-card-processing...