We do still have mosquito abatement programs too.

I know here in Louisiana the local abatement program has trucks that spray synthetic pyrethroid chemicals (resmethrin, sumithrin and prallethrin) at night and uses an organophosphate (naled) from planes.

And residents are routinely urged to be aware of and eliminate standing water on their property.

We just don't use DDT anymore -- we almost wiped out our state bird with it.

Edit: I have to wonder how much housing changes also mattered. With modern air conditioning and well sealed homes it's fairly rare to have mosquitoes in the home at night.

Malaria does not spread from human to human, you need infected mosquitos in the mix. So you would need to get a breeding population of infected mosquitos biting humans for it to start spreading.

The answer is malaria eradication efforts never stopped neither in US nor in Europe. It's just as routine as renewing asphalt roads or maintaining sewers

Only explanation I can think of is that the malaria from elsewhere is different somehow, worse adapted to US conditions.