Iirc the main reasons at the time were the economy, fuel shortages and Iran.
Which is kind of ironic.

Probably the main difference with today is that there's no fuel shortage, but is expensive af, and the previous economic malaise was caused by staglation (high unemployment and high inflation).

The post-Vietnam war and post-Watergate 70s was a bad time to be around for most, and Reagan used this as a pretext that Keynesianism wasn't working and that small government (i.e. tax cuts for the rich, deregulation*, low social expenditure and somehow imcreased defense expenditure) would save the economy, but we know how the story goes.

*Deregulation had started with Carter.