The idea was mostly that X.500 was too complex because it also dealt with user authentication and updating the directory data. During the initial development the ability to update dircetory data was then added back and then LDAPv3 allowed for non-password authentication using mechanism that is at least twice as complex as what was in original X.500.
Not exactly. X.500, like all the ISO networking standards had two significant problems : 1. It was defined to operate over transport layers that nobody used. 2. Its standards documents had to be purchased from the ITU, so approximately nobody had access to them and approximately nobody could participate in discussions. LDAP provided a vehicle to tunnel X.500 into the IETF standards process, and onto TCP/IP networks. In the process various things were simplified, but the raisons d'être were those two things.
1) was rectified pretty easily, there were X.500 implementations on top of TCP/IP. 2) was definitely annoying. They seem to have loosened restrictions on access to the specs these days.
X.500 (and X.400) got a lot of pushback in the IETF crowd because of the use of ASN.1, while all IETF protocols of the era were still plaintext. It's been enormously clear from then till now that binary protocols are superior in terms of network efficiency, and everyone has tools like wireshark / etherpeek / whatever rendering the human readability of packets a moot point.
