Playing Devil's Advocate, there can be good reasons why e-mails do not get replies.
In addition to what Matt said, we frequently get questions along the lines of "send me everything you have about Korea." We also get a lot of messages that over the years I have learned indicate a very high-maintenance customer (with the emphasis on the high maintenence and, in the end, the customer part often doesn't work out and even if it does, it is rarely worth the effort). So like Matt, I know we may be losing a few customers. But I prefer to focus on the better customers.
We also get messages that look like they were written by 4-year olds: no puctuation or capitalization and either attrocious spelling or SMS-style wording. These have a low priority for answering.
And we sometimes get extremely vague messages like "There's an error on your site" or "I can't log on." Over half the times that I have responded asking for more information to help them fix the problem, I never hear back. So these types also have a low priority for answering.
Lastly, I have replied to a number of messages sent through our contact form that came back because the person entered an incorrect e-mail address. Other replies never got delivered because of SPAM filters turned up way too many notches.
So if you want an answer when you contact a site, be professional, be specific, be clear, and be careful.