HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

Could you please explain why the project is not available on GitHub?
According to the author, the amount of work has already exceeded the capabilities of one person. So why not allow other enthusiasts to create their own PRs and report bugs in the project itself?

Firstly, because the HD source code is a very powerful base for any kind of cheating. And I believe that making it open would negate all the work on features for fair play, such as password-protected saves.
(Although, of course, there is already plenty of room for cheating here)
Secondly, HD contains code or developments from other developers who, for one reason or another, strongly oppose their openness.
Thirdly, the HD author is greedy for fame.
And finally, the author is now making money from the project, so there are not many options.