That is also a disadvantage, Github has a lot more grifters, people submitting fraudulent and malicious PRs, issues spam. In similar vain as "everybody is on windows" and Linux not being targeted by malware as often.
If a person really cares about your project and wants to improve it and not just boost their own GH stats - creating an account takes no time or they can always send you patches via email.
Also: GitHub is so established that for many people git and GitHub are the same thing.