It could be any number of things. People are busy and don't always have time to check their emails, they may over look your message or decide that they want to respond later when they have more time. And there will be, of course, those who don't believe your skills are up to their standards. Unfortunately, there are a lot of people who don't have the professional or even the common courtesy to respond.
Don't let it get you down. It's a numbers game so if one photographer doesn't respond, try some one else. Persistence pays off in this industry.