This question was recently discussed on Quora. I wrote an answer there, which I have expanded on for this post.
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 a 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.
The natural question that comes out of the above discussion is: As a startup CEO, how do you decide whether to keep pushing on with a new startup or throw in the towel?
In this context, I want to point you to a case study: Crossing the Desert, Hitting the Jackpot: Taboola CEO Adam Singolda.
Adam went four years with no revenue, and somehow managed to stay afloat.
Year five, Taboola had $1M in revenue.
Year six, $100M in revenue.
Today, the company has over $200M in revenue, and is kicking ass in a new category called Content Marketing.
This being said, pure persistence won't do it. You need to be creative, experimental, AND persistent. What isn't working, and what is working - these two questions need to be your constant companions.
Photo credit: gerry/Flickr.com.