What is the biggest road block that prevents a supply chain from working together to become more Lean and optimized?

I have found that often suppliers don't like their customers making such suggestions due to the fact that the supplier wants to appear as the expert and authority on what they do and how they do it.

Another issue is Money. Suppliers don't like leaving money on the table for their customers to take. They feel cheated when this happens.

It takes careful negotiation and clear communication by the customer to help the supply base see how implementing Lean strategies will positively influence their business and help them grow. One motivation could be through making them a preferred supplier. etc... It will also help them if they see high value in their relationship with their customer.