I think the problem is that it wasn't low maintenance. The moderation of the links for malicious links was becoming an increasing drain on resources. It's the same problem most large tech services run into eventually.
To bring down the point, here's the source code for that link shortener: https://github.com/technoweenie/guillotine. It was last updated in 2015. If you don't believe that this is the source code, this was directly linked to in its announcement: https://github.blog/2011-11-10-git-io-github-url-shortener/. If the extreme lag (in an attempt to save the links) is indicative if its backbone, it's probably just running in a single server, which is very likely to be horribly outdated. It might not be due to the cost of removing the malicious links, it might be that Git.io as a platform has a cost in of itself (even excluding common things like domain and hosting costs).
You just proved the point.
https://github.blog/2022-04-15-security-alert-stolen-oauth-u...
“The applications maintained by these integrators were used by GitHub users, including GitHub itself.”
* Technically the links are not publicly listed, which might jeopardise some obnsscure but technically-available repository, but it doesn't store private data.
From the original post: “due to the security of the links redirected with the current git.io infrastructure”
What does “security of the links” even mean? Disclosure? Tampering?