Laws vary, and they still have to build out the infrastructure to support whatever bank they use in a given country and the laws and implementation differ dramatically.
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.
But why do they need to use a bank in Romania and create some infrastructure for that? Can't they just use some bank in eurozone to gather money from customers and send out payments to the users in their local Romania bank, or something like that? There are payment providers which do that.
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.
