Network demand seems to be the best answer. There were key network switches and fibre that was located at those sites which was destroyed during the events. I remember nothing was working or loading. News sites went to plain text with minimal content during this period.

The book "The Internet Under Crisis Conditions: Learning from September 11" appears to deal with this fairly well: https://www.nap.edu/read/10569/chapter/4

This was the era where a major computer consultant (IBM) failed to provision for the Olympics: https://archive.fortune.com/magazines/fortune/fortune_archiv...

CNN had to revert to a bare bones HTML page because their servers and Internent connection couldn’t handle the load of everybody reloading as things unfolded.

Combination of loss of a lot of connectivity in New York, with massive amounts of increased load on the databases of sites from everyone wanting to know what was happening

It was nearly 20 years ago, but from memory the BBC did load occasionally, really really slowly