Because many bought simultaneously with new Apple releases and have been waiting to see the next major version. In my case our MacBook pros are > 3.5 years old,

Because many bought simultaneously with new Apple releases and have been waiting to see the next major version. In my case our MacBook pros are > 3.5 years old, which is too old for my comfort level; I expect major failures after 3 years that aren't worth even one day of lost productivity.