Simply because its Dutch, and its not a US VC funded company. There are many companies that are much larger in scale when compared to the US counterparts, but are virtually unheard of in the US.
People in the payments space know of Adyen very well. Just like people in the P2P space would tell you (as an analogous example) that Alipay (product) is huge (much larger than PayPal (product)) yet available only in China and a handful of countries.
People in the payments space know of Adyen very well. Just like people in the P2P space would tell you (as an analogous example) that Alipay (product) is huge (much larger than PayPal (product)) yet available only in China and a handful of countries.