Most likely because of its short-term oriented view.

The weakest point of the methodology, in my opinion, is the excessive emphasis on quick validation and pivot. If you don't have internal conviction, you look for instant validation. But often, especially if you have strong vision and internal conviction about market, a product, a direction in which you want to take your industry, you won't be able to score a quick validation. You would need to give yourself and the market some runway. The Lean Startup principle that Eric Ries espouses ignores this whole line of thought.