Because the UK was a (or even the) driving force behind that attempt, so Ms. May would probably be very happy to sign a quickly-relabelled draft - just s/EU/UK/g and off you go. After all, she will need a quick victory after the breakup, and a TTIP clone would be an easy one. TTIP drafts were always pretty unbalanced towards US interests, so Mr. Trump should be quite happy to sign it as well.

It would be very ironic, in the Morrissette sense: the spectre of TTIP ratification was used by Brexit supporter in the referendum campaign as a reason to leave the EU, and now we could end up being the only country that actually signs it.