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I think the main reason is because Windows XP included a basic MSN Messenger client enabled by default in 2001.

Indeed, ICQ was vastly superior. In fact, in my opinion, many of its features (such as the discovery features) have not been equalled since them. It was quite traumatic to me when it was replaced with MSN Messenger.

AOL bought them. Slow burn.

ICQ is now owned by the Russian company behind Mail.RU. AOL sold it for $187.5 million, less than half what they paid for it:

https://techcrunch.com/2010/04/28/aol-sells-instant-messagin...

in my circle of friends it was because aim let you pick a username instead of a number

I remember that they bloated up their client so much with menu bars, icons and later ads that it was not snappy anymore and required a lot of memory. So people where looking for alternatives.