The CRISP working group considered both an LDAP-based proposal
and IRIS during its initial development. If you check the archives,
you'll see the extensive discussions of the pros and cons of each approach
(http://www1.ietf.org/mail-archive/web/crisp/index.html). At the time
of that discussion, the RIRs were not interested in using CRISP, so that
decision was made based on domain name registry requirements.
The RIRs interest in re-using the work of CRISP came after IRIS had
already been chosen. The decision might have been different in both
were being decided concurrently, but that's not my personal impression.
You may also wish to recall that LDAP was standardized in the IETF
and had active working groups in the Apps Area as recently as last
year (LDAPBIS; LDAPEXT closed earlier, as did LDUP). I don't think
it was rejection of external forces that drove the decision.
Full disclosure: I was chair of CRISP at the time of the decision discussed
above and later AD for both CRISP and the LDAP working groups. That
probably makes me a biased observer. Your mileage may vary, and
past performance is no guarantee of future results.
Ted