Because of a lot of things. From graphics side RTX and DLSS left AMD catching up (even if RTX isn't really that big of a deal now), then there was Nvidia cards being better at crypto mining and now it's Nvidia cards being better at AI computation + Nvidia pivoting into AI hardware space..
If you want to boil it down to the undeniable, it's that Nvidia is just better at marketing. Everyone knows what Nvidia is doing. What is AMD doing? Besides playing catch-up to Nvidia.