Because you might be buying at peak of "rapid growth".

If fundamentals are good, no matter how irrational the market is, you still win in the long term: you can just wait until you return investment through dividends, or share price is corrected via buyback.

But if you aren't looking at fundamentals and projections, only at "track record of rapid growth" and "well-managed"-ness, you depend on investor moods: growth might continue, or price might fall once investors realize that they can get bigger returns elsewhere.