To modernize a running system, you have to duplicate everything so that you can someday do a switchover.

For commerce tech, you switch, see what breaks, switch back and adjust for next try. You may have lost some sales. But probably just inconvenienced people.

When doing travel systems, something breaking is likely a lot of deaths.

A wide variety of decades-old aircraft, and even newer aircraft are often modernized versions of ancient aircraft models. All of them built and operated with the motto "if it ain't broke, don't fix it", because every change brings risk (and costs money).

There was also a lot of resistance to the idea that every aircraft should broadcast its position, heading, etc. because of privacy concerns. And as long as there are holdouts the tower can't rely on the information being available

Have you read the various reports that are available right there on the web page?

Large infrastructure projects always take decades.

Look at the railroad development in EU for example. You have to invent a lot of tech (signaling, communication between trains, interoperability with old systems), build a lot of infrastructure, train a lot of people and roll it out at scale with the goal of zero accidents.

It is just impossible to do quickly.