It was MSN messenger that came preinstalled with any new pc that slowly killed it, because a lot of people got their first computer in that period and thus only knew the Microsoft one. ICQ users slowly started to have a secondary MSN Messenger account (the days of Trillian) but that was it, once people have 2 messengers apps AND most of their friends are on the new one, they just slowly abandonned the one with less contacts.

I personnally didn't even knew there was a Yahoo issued Messenger until this comment.