> why wouldn't the trump admin use the tariff act of 1930?Probably because they knew it'd be a losing argument.
The court that authored this slip opinion would be unlikely to be persuaded by such an argument, for two reasons.
First, the opinion spends several pages applying the non-delegation and major questions doctrine. Based upon that discussion, I am pretty confident that this court would've found your interpretation of the Tariff Act of 1930 to be an unconstitutional delegation of congressional power. A similar question was asked in the Nixon admin; grep for "Yoshida II" in the PDF.
Second, even if your interpretation of the Tariff Act of 1930 were found to be constitutional by this court, the argument you suggest would still hit a brick wall.
Around page 35, the court cites the President's executive order in finding that the WaRTs are addressing a balance-of-payments issue. The court then notes that Congress specifically delegated narrower presidential authority for actions addressing balance-of-payments deficits. So even if the president were allowed broad emergency powers, and were allowed broad discretion in defining what emergency means, that finding would be irrelevant, because Congress has specifically curtailed the delegation of authority to the President in the case of tariffs addressing balance-of-trade issues.
Specifically, the opinion notes that Section 122 of Trade Act of 1974 limits Presidential authority to response to balance-of-payments problems, such as "a 15 percent cap on tariffs and a maximum duration of 150 days".
The conclusion also specifically addresses your question about emergency powers: "Congress’s enactment of Section 122 indicates that even “large and serious United States balance-of-payments deficits” do not necessitate the use of emergency powers and justify only the President’s imposition of limited remedies subject to enumerated procedural constraints."
(These are not my opinions; I'm just applying the legal reasoning in the slip opinion to your question.)