The reason? At about that time I had a huge man-crush on Calgary's Joel Otto. A physically dominant, extraordinary defensive center with incredible size and great faceoff ability. Every team wanted Otto. And, I believed, Toronto had drafted the next one when the draft Thornton ahead Stu Barnes and Bill Guerin.
Thornton went on to become a bit of a poor-man's Otto rather than the next dominant defender. He was outstanding on faceoffs. He played with a different variety of toughness - he was never chippy or cheap. He was a real solid player, though prone to both injuries and bad penalties.
Though he had decent skating ability - strong and balanced - he was not fast. He was smart positionally on the defensive side. Offensively he never really was a threat, except for one season in San Jose where he played a lot of left wing alongside his superstar cousin, Joe Thornton. Many people would expect more offensive production from a 3rd overall draft pick - one who was traded as a key part of the trade to Edmonton for Grant Fuhr. But I certainly would not consider him a disappointment.
In fact, even though he was a favorite of mine, even I was actually quite surprised to realize he survived parts of 17 NHL season, totalling nearly 1000 regular season games. He spent a long time in obscurity on 4th lines in Toronto, Edmonton, Montreal, Dallas, San Jose and Los Angeles. It all added up to a very decent National Hockey League career.