There has been lots of talk by Trump and his advisers about renegotiating NAFTA, or if that doesn't work withdrawing from it. With TPP, by contrast, they mostly just talk about abandoning it.
In terms of the substance of NAFTA, they seem to be focused on issues such as VAT rebates. But as I said here, that's not really a NAFTA issue -- it's much broader.
So, if they want their proposed changes to trade rules to have a real impact, why not raise them as part of a renegotiation of the TPP? (And keep in mind that the TPP itself was, in part, the renegotiation of NAFTA.)