Highschool, your question is somewhat complicated. A simple answer would be that if you have a legitimate reason for not attending a tryout the coach should accept it.
But the question you have to ask yourself because the coach may be asking himself is, does your reason indicate that you are not truly interested or you are unwilling to sacrifice for the opportunity? Are you saying that something else is more important and will he agree with you?
For instance, you give distance as an example. Well, if the school is 150 miles away and you really want to play there, he might expect you to find a way to get there. 1500 miles might be different if it's not an official visit.
Same for previous commitment. If you are going to a tryout at another school I would assume you are more interested in that other school and so will the coach, most likely. If you are playing with a team that you are committed to a coach might understand that. But he might not. He's got his job to do, too and not making it to his tryout might make it more effort for him to look at you.
If you can't go because you are injured, that tells the coach you are injured. Injuries and durability are definitely considerations for a coach.
Pretty much anything you say is open to interpretation by the coach and different coaches will have different responses. Some might write you off then and there because that's how they are.
Others might have tremendous respect for you just because you get on the phone with them and explain yourself like an adult.
You tend to learn something about the coaches in these exchanges, too. Some play games, some don't. Some are straight forward, some not. Some you can't tell.