I imagine they probably started getting a lot of negative attention.
However it is supposed to be from the copyright owner [17 USC 512(c)(3)(A)(i)] and if it isn't, it can be ignored [17 USC 512(c)(3)(B)] and makes the person who does it liable for damages [17 USC 512(f)(1)]. Although that's probably pretty hard to use in practice.
I was being dumb and am wrong. This is YouTube's internal system for copyrights not DCMA.
Apologies.
Old comment below...
You can read up on how to submit DCMA takedowns for YouTube here -> https://support.google.com/youtube/answer/2807622?hl=en
The original uploader of the video submitted the DCMA takedown request.
YouTube's internal copyright takedown system is just that: Internal. DMCA is a legal avenue that you can use against YouTube and their users, but it wouldn't be via their internal copyright take down system.
It is in YouTube's best interests for you NOT to use the DMCA as there are avenues where it can leave YouTube themselves civilly liable (OCILLA[0]), so they work hard to funnel people into their internal copyright system that doesn't expose YT.
[0] https://en.wikipedia.org/wiki/Online_Copyright_Infringement_...
Edited the comment to note my error.