The IRS (god bless their soul) and the state tax systems have set in place guidelines for when a company needs to collect interstate taxes. If you placed an order with us before and weren't charged tax, but now you are it's because your home state has now seen that we are doing too much business in your state for their liking and now want to collect sales tax.
This is of course the 'short' version of the reason why, but because we are a legitimate business we prefer to not attract the government's attention unnecessarily.