The most likely is that they have to charge VAT in Europe, but they don’t charge sales tax in the US. (Also, VAT and Sales Tax work differently – in Europe, prices are generally quoted with VAT already included; in the US, prices are generally quoted without sales tax.)
However, it’s also possible that they have additional costs in Europe, for reasons that could vary from “currency market risks” to “legal compliance with customer records risks.” They might choose to charge more because they’re taking on those risks.