It's not an official rules, it's just something that has typically been done in impeachment hearings. Pelosi could probably just deny Republicans the right to subpoena in an official impeachment inquiry as well, but that would be yet another thing she would have to defend.
At least McConnell defended his actions of not giving Garland a hearing. You don't have to like his excuse, but he gave one. He didn't just use some lame tactic like Pelosi is doing.