Because that's redundant. The purpose of both is to hide your IP address from whatever website you're connecting to. With a VPN, you send your request to another computer, which forwards your request to the end website. With Tor, it's the same idea, except there are three separate hops instead of one, and the computers involved are randomized. Tor is a much stronger method of preserving your anonymity, because you aren't trusting only one person to keep your identity secret. So there's no reason to send your traffic to a VPN before you send it to Tor, since Tor's security is already bulletproof. It'll just slow down your requests even further.
If you want to hide the fact that you're using Tor from your Internet Service Provider, you should be using unlisted Tor bridges.
sudo wrote (edited )
(That was supposed to be greentext arrows, but it didn't quite work out.)