Just why (or when) isn't clear. You might have to screen for an option if you prefer to look at YouTube using the Leanback interface on a home theater PC. YouTube calls users to the Leanback web portal that this variant "will be leaving shortly."
Of course, this will not prevent them from looking at YouTube, but they must go to a computer where they want the most easy-to-navigate interface. We asked for a remark from YouTube.
Why YouTube might take the web app at this point is not evident. Android police suggested this might be an effort to prevent phone manufacturers from circumventing their app via a browser.
However, as the internet version becomes less and less essential, it could also be just a question of moving company resources. With a mainly identical interface, many individuals now have access to intelligent TVs, consoles and media hubs.
The internet version is redundant in those instances. This may assist YouTube to put more effort into native applications.