Kenya had sovereignty in production, but not in capital - which is what was needed to climb up the value chain.
Historically, Kenya's largest capital market has been the UK, and as such Kenya's administration was unable to break the oligopoly of coffee purchasers who were British-Kenyan dual nationals and descendants of British settlers.
The same kind of land reforms that former colonies like Indonesia, Vietnam, India, etc were able to drive did not happen in Kenya.
Now corporations from those former colonies are gobbling up land and production in Kenya and other African nations, but domestic African (excluding South Africa) continue to remain on the back foot.