They simply sell out as fast as the supply hits. Amazon had a batch in today, its already showing ships when in stock though. No, X299 is not new at this point and there are plenty of the other boards to be found. The Extreme however is always late to the party and initially hard to come by. They are not mass produced and made in small batches. Typically a vendor gets 10-15 of them at a time and they are gone in a matter of minutes.
There are two ways to get it. Search incessantly or go to nowinstock.net and sign up. It does the legwork and you will get an email alert. I decided I wanted one and just did a google search at least hourly for 3 weeks before they showed up at Walmart online of all places.
There are a few to be had right now but some of them are gouging. Ebay always has them if you want to pay through the nose, these scalpers is why they're in short supply.
Another thread on the same topic already exists
https://rog.asus.com/forum/showthread.php?96505-Rampage-VI-Extreme-availability
“Two things are infinite: the universe and human stupidity, I'm not sure about the former” ~ Albert Einstein