That's like asking:
- Why is the iPod different from all the mp3 players on the market?
- Why is the iPhone different from all the smart phones on the market?
- Why is the iPad different from all the tablets on the market?
None of these questions are easy to answer in full, if they were Apple would have more serious competition. The simple answer is:
Because the overall experience of owning them, for the average person, is better than with the alternatives.
I suspect the current hype about the Nest is due to two reasons: 1) Apple people made it, and Apple is newsworthy right now; 2) it doesn't look like other thermostats, it looks easy to use and unique. I think that right now the Nest's aesthetic is getting it traction, time will tell if that's enough to get it sales.