I think this is the number one reason. The issue is that the prices are too high for perceived value.
I buy a fair number of non-fiction books and loved when ebooks emerged as a viable way to buy a book at what I perceived to be a reasonable price. Now that the agency model has been in force I've just shifted how I buy ... I buy used books from Amazon.
All the books I used to buy as ebooks the publisher was receiving revenue. Now, they don't receive any money from my purchases.
The book publishing industry still has a window of opportunity to transition to ebooks under their own direction, but it's almost closed. See the recording industry.