From https://faqs.ankiweb.net/why-does-ankimobile-cost-more-than-a-typical-mobile-app.html:
Apple does not make it possible to set different prices for different countries. Apple sets the price in each country based on the local taxes and costs of doing business there.
Please also bear in mind that the price in the US is the price before state sales taxes are applied, so the difference may not be as as much as you think.