>>pjc50+RJ
It was far from the only effective solution. Probably just an easy path for someone who has no idea about DDoS attacks, but influenced by advertisement and propaganda. Volumetric attacks don't actually require centralized global intermediaries to mitigate, there are other ways to do it, and Layer 7 attacks are even application specific and should be handled by applications or by someone running them who understands all the specifics, but most definitely not by a global intermediary, as unaware of the specifics intermediary will reject plenty of legitimate traffic. And blocking Tor to mitigate Layer 7 attacks is pretty silly.
Also, it was only up to like very early 2000s when researchers of decentralized systems mostly ignored the existence of malicious actors, but later everyone became well aware of them and started considering how to deal with them.