When a company becomes successful, salaries increase and company reputation improves.
This obviously increases the incentives to get into the company, which in theory should be good: more people want to work there, so the company has more options to choose employees from.
But this pay and reputation increase disproportionally attracts the kind of people that don't really care about doing a good job or creating an exceptional product, but only care about the money and status.
If the company lets these people in, they will slowly destroy the culture that made the company succesful in the first place. The general feeling goes from "how do I contribute to the company creating the best product ever?" into "how do I get a promotion as fast as possible?"
But that's just my personal hypothesis that isn't supported by hard data.