We have customers in many countries, of which the US is our largest market. We price our plans in USD so that we are in the local currency of most of our customers. This allows our pricing to remain stable and consistent, rather than fluctuating with exchange rates.
As software that serves customers all over the world, we ultimately had to make the decision to price our plans in a currency that carries the least worldwide "friction". For us, this means that the US Dollar offers the best chance at being as fair as possible to all customers.