Muddiest Point: Lecture 3

As usual, there really wasn't much confusing about this week's lecture, so I'll just ask another question out of curiosity. With regards to the history of different versions of Windows, why is it that so many after 95 have had such short lifetimes? It seems that NT, 98, 2000, and Me were each considered top-of-the-line for only 1-2 years each. What's different about XP that has allowed it to succeed despite so many earlier versions having failed?