Because Pixel is not available in most countries, including more than half of Europe. I had the Nexus 5x, I wanted a Pixel and the only option was the gray market at double the price.
It is Google's decision. I guess it may be related to consumer protection laws in Europe that sets the minimum warranty to 2 years; I had 2 defective Nexus 5X (boot loop), fixing or replacing it is costly for Google. I replaced one on warranty, it was bought locally, the other one was bought from USA and I could not RMA it.