Essentially Congress got tired of having to do actual work because it was taking time away from them soliciting bribes from corporations, which is the profitable part of the job, so they abdicated most of their responsibilities to the president.

Constitutionally the President doesn’t really get to do much except work out the administrative details of everything Congress has ordered him to do. Since Congress abandoned that responsibility the president has a lot more control.

Trump still doesn’t actually have the authority to do most of what he’s doing, but he’s abusing emergency war time regulations as justification despite this not actually being an emergency or war time. Everybody is just too chicken shit to call him on his bullshit though because then they’d have to own up to all the times previous presidents over the last few decades abused the same loopholes.