It is simple economics. It takes a lot of money to launch an assault on an entrenched market leader. If you know any venture capitalists willing to invest 100 billion dollars or so to finance the renewal of competition in the Wintel market against Microsoft you should have them get in touch with Apple. Unfortunately no one appears to be willing to finance a competitor to Microsoft and who can blame them since such a battle would require quite a few years to show a return and has no guarantee of success. I have little doubt that if Apple could afford such a battle they would willing to phase out the computer hardware side of the business for a second chance to overthrow "the power." (This time MS not IBM)
That's my take on it at least.