Because the Senate is the only body with standing to challenge a "trade agreement" as being in violation of the Treaty clause. And the Senate no longer wants this power.
Ted Cruz, as a member of the Senate, could mount a campaign to restore the Treaty authorization clause of the constitution, but instead he signed on the TPA, which removes even the power of the cloture to stop a bad treaty "trade agreement."
Yep.
Although I’m not aware of even one senator opposing any trade negotiating agreement based on the treaty approval clause.