He's been on this particular line since his first presidency. From a 2019 interview quote:

I said, “You don’t use steam anymore for catapult?” “No sir.” I said, “Ah, how is it working?” “Sir, not good. Not good. Doesn’t have the power. You know the steam is just brutal. You see that sucker going and steam’s going all over the place, there’s planes thrown in the air.”

It sounded bad to me. Digital. They have digital. What is digital? And it’s very complicated, you have to be Albert Einstein to figure it out. And I said—and now they want to buy more aircraft carriers. I said, “What system are you going to be—” “Sir, we’re staying with digital.” I said, “No you’re not. You going to goddamned steam, the digital costs hundreds of millions of dollars more money and it’s no good.”

It's because the president is 12

He seems to think that magnets stop working when they get wet. He's made numerous comments to that effect going back years.

He has the -tism for early 19th century steam warships.

He's just an idiot. The US already has a Ford class aircraft carrier that has conducted tens of thousands of sorties using EMALS since October 2023 in the USS Gerald R Ford. The US successfully launched an F-35C from a ground based EMALS catapult 12 years ago. The USS Gerald R Ford doesn't operate F-35Cs as part of it's air wing, because of a procurement decision to equip older Nimitz class carriers like the Carl Vinson and George Washington with the support equipment for F-35Cs first. The second Ford class carrier, the USS John F Kennedy, has already tested it's EMALS in trials. The US is not going back to steam, Trump just doesn't like magnets for whatever reason.

there was an article here recently about china using some kind of electromagnetic launch system so it's probably just being anti-whatever-china-does

Reckon it's cheaper