Because Linux still isn't ready to roll out for the average enterprise office user. My mother, father, wife, and sister all have higher ed degrees and fruitful careers and can't realistically use a linux machine - even today. Linux' open nature has been it's greatest achievement, and it's worst enemy. Lack of commonality or agreement on package sources, etc., has led to an ecosystem that's all but inaccessible to the Real World.

I love it myself, but until you can walk into a Best Buy and a quarter of the options are Linux machines that the sales people can show off as well as the mac airs and surfaces, it's going to stay for people like me.

And that's just the home world. Trying to bring business into it? Unless your staff is purely development oriented, you now have to retrain every one of them (and probably most of the developers). Linux for wide spread average business user use is a non starter.