Because they are considered to be the best laptops on the market. They only recently went from IBM and were sold to Lenovo, a Chinese company. A lot of trust was built up with the brand.
Bios Magazine got their hands on one of the latest Lenovos..."it's definitely not a ThinkPad."
http://www.mobilemag.com/content/100/334/C7154/
Lenovo...didn't quite reach the standard that long-time IBM users are accustomed to. The build quality is average at best, the screen is low resolution, and the notebook itself is "aesthetically challenged."