Question: Why is mentioning 2, 3, ... users (by `@user`) in one comment not allowed?

What's the bad thing could happen if Stack Overflow allows it? I didn't understand.

Answer: 
Comments are to be used to **talk with one person to clarify some matters** which the person posted on your question/answer.

Otherwise, Stack Overflow users would abuse the comment section to bring attention to their Q/A, which would most likely be perceived as very negative. Example:

> 
> @abdulla @Robert\_Harvey @DMaster please check my question
> 
> 
> 

We also can't ping mods. Please also read: [How do comment @replies work?](https://meta.stackexchange.com/questions/43019/how-do-comment-replies-work)
Comments have only two sanctioned purposes: to clarify a post, or to ask for clarification. So ask yourself: how would talking to two people at the same time advance these purposes? 

There are plenty of unsanctioned uses, however. It's just that we don't feel the need to encourage those uses.