As far as the marketing goes, it is a larger trend. See Fedora Workstation/Server, elementaryOS. In fact, on the CentOS homepage there is nary a mention of Linux.
Maybe that's unfair.
If you look at the whole product, there's much more to it than Linux. Linux is 'just' the kernel, I doubt the average Joe cares what kernel his device runs. Most probably don't know that their Android phone is powered by Linux, too. Also, this leads to the old "GNU/Linux" discussion. Where would Canonical stop acknowledging important parts of the OS? Ubuntu GNU/Linux/systemd/libinput/Mesa/Qt...
I'd also love to see Canonical mention that Ubuntu uses Linux, but I understand that for their product and their target group, it doesn't really matter (and it may seem more important to push forward the brand "Ubuntu").
We know that Ubuntu is "Ubuntu Linux" and Linux is "GNU/Linux".
I'm not trying to make a point here, I just wanted to give some context to someone asking a question. I'm not an Ubuntu user, but I'm not an Ubuntu hater either: the distribution I use (Linux Mint) wouldn't even work without Ubuntu.
That being said I think they somewhat deserve the bad opinion the Linux community has about them: Canonical decided to play solo on several critical subject instead of cooperating with others, in particular I'm thinking about upstart (a competitor for systemd) and Mir a competitor of wayland).
Their marketing is also pain point, because they brand everything as Ubuntu and don't refer to Linux at all in many of their statements (for instance, you cannot find a single occurance of the word «Linux» on their landing page[1]).
[1] https://www.ubuntu.com/