Slow supplier response times is a real frustration and you're not alone in feeling that. I would say it's one of the most popular topics we get complaints about, supplier simply not responding to inquiries. Email is a convenient way of making inquiries, but we've found that it can also be the least productive when dealing with suppliers initially.
I don't really have an answer as to why many have such delays and then others simply seem to ignore email inquiries altogether, but it seems to be the reality for some reason. The one way that does work is phoning them directly, they'll always pick up the phone, it just seems to be emails that many simply don't consider to be genuine inquiries.