A big factor is that ICANN is comprised of multiple stakeholder communities of competing interests that have to come up with consensus to make new policies. Refining the model of what is published in the WHOIS has been the subject of working groups in ICANN for over 10 years, but consensus was never reached because you had a huge spread of opinions that never converged. Privacy advocates argued for no WHOIS, whereas interests from law enforcement, security research and intellectual property arguing for full disclosure.
