I always get text messages from my mobile phone on my tablet. Why not?

If both the tablet and the mobile phone are Apple's, and the account number in the app store is the same, then if someone sends you a message with imessage, you will receive a short message (both devices are networked). Now, if someone sends messages in other ways, your device is not connected to the internet, and your Apple account has changed, you can't receive text messages at the same time.