Here’s some quotes from this CNN article:

The breakdown was caused by failures in the copper wiring that transmits information from Long Island to Philadelphia, a source told CNN.

“What we use today is copper wires. We are one of the last institutions, last businesses, that actually use copper wires,” Duffy told reporters Thursday.

“We should be using fiber, but it’s copper. We use radar from the 1970s, some of them are from the 80s… So this technology is 50 years old that our controller has used to scan the skies and keep airplanes separated from one another.”

Some controllers still use floppy disks to upload data onto computers that date to the early 1990s, according to experts.