I have bought Volos, and want players in my campaign to be able to select volos races, but they don't appear for them. Why is that?

Do you have the Master Tier subscriptions and enabled the sharing?

Also, they are using the character builder from the campaign? The Volos material will not be available from the personal character builder. 

I don't have a sub. That seems to be the issue. A monthly fee just to share some basic content that I've already paid for multiple times seems crazy to me.

One of your players can have the Master tier and hence activate the sharing.