The problem is that Stripe is still not usable in the half of the world, including my country, Romania.
Romania has some problems and companies like Stripe have to decide whether it is worth it to them to open up to a country that would in turn most likely (at least initially) yield relatively small volume in real transactions and a disproportionate amount of volume in fraud.
From the Romanian point of view this sucks because those Romanians that would make something nice and use this to move themselves and by extension Romania forward don't get the chance to do so.
Personally I think the initial problem is with Romania, if rampant fraud and corruption were not so widespread then other service providers would feel more secure in opening up to the country and giving it a higher trust level.
But given the way Romania practices politics I do not see any major improvement happening any time soon, which is a huge pity. I love the country and I love the people but the business climate and the government are terrible.
Actually, I don't exactly understand what Stripe do to "prepare" for the launch in a country? They are expanding very gradually, and there are payment processing companies which basically blanket-cover almost whole world. For example Stripe still marks Germany, Austria, New Zealand etc as "preview", and they were not available some time ago, but fraud and corruption definitely is not a problem there.
For now, adding countries that have the best combination of low fraud and least difficulty or barrier to entry is the best choice. Romania doesn't have the former relative to Germany and Austria, as such, it's not on the list.
There are a lot of laws involved when you wish to become a financial operator and that can be a real pain for Stripe to handle in each country separately. PayPal managed to do this. For example PayPal operates as a bank in Europe to be able to conform to laws.
The only real alternative is PayPal at this moment.