A KLM spokesperson (Mr. B. Koster) issued on March 18th a statement to the Dutch news agencies (after rumours in the Dutch financial community that Ryanair was backing out of the Buzz sale) that KLM and Ryanair couldn't agree on the price Ryanair has to pay. Furthermore KLM wasn't sure the UK competition authorities could clear the sale before the deadline of April 1st.
Thus it looks as if price is the most important issue