Could be out of stock in the country you checked, but not discontinued. Also, could be they only made one or two runs, and they are finally sold out.
I still see options to order many models in the US here: https://www.omegawatches.com/en-us/watches/de-ville/hour-vision/catalog
I'm also in the US. The link you provided worked, but what if you navigate to the De Ville on the top of the website, like below?
Thanks, I found the issue. Even though English was selected in my browser, and I'm logged in as a US user, it didn't select US as the Country / Region. Once I selected US, the Hour Vision section appeared. I do find it strange that it's not available in Switzerland, Australia or UK, once you select those Countries.