It promotes the idea of a magic number, and that hard work and time leads to returns (not true for the majority of startups). It encourages thinking like "Man I'm at the 950 hour mark, I'm almost there!", or even worse, planning one's product milestones around time spent.
One of the main attractions of the 10k hour rule is that there are few external stimuli that can negatively affect your learning/training.
If I decided to invest time in taking up the trumpet, I don't have to worry about competition with other budding trumpeters or whether BrassCrunch or Spitter News have effusive posts on my latest performances. I won't have to determine which percentage of the public 'gets' trumpets and plan how my playing should appeal to them.
With proper practice one hardly ever goes backward -- ability increases monotonically give or take a few plateaus. With startups you often go backward as you try to figure out your product-market fit and so on. Userbase size or revenue would not increase over time the way ability does when it comes to personal improvement.
1. I haven't actually read Outliers but I'm familiar with its thesis. If it's anything like Gladwell's other books, I recommend The Talent Code for better treatment of the subject matter. Norvig's http://norvig.com/21-days.html is also great reading.