"X.500 is too complex to support on desktops and over the Internet, so LDAP was created to provide this service 'for the rest of us'" - https://en.wikipedia.org/wiki/X.500 (aka. DAP)
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.
I understand the case for the irony, but it's one of those situation where a tecnology can do say 1000 different things but 99% of the people only need like 5-10 of them.
It's the same with OpenSSH, Apache httpd and and many more tools. Take OpenSSH as an example: it can do a lot of little-known things (SSH certificate authority, SOCKS proxy, port-forward, signing stuff and verifying signatures and so much more)... Yet like 99% of the people will just do "ssh somehost" and be done with that.
I remember a story of a X400/X500 based email system taking two hours to deliver mail within a single machine -- around 1990 or perhaps a little earlier.
By comparison for one of my first emails I got a reply within 5 minutes from France. I'm in Australia.
> size slapd
text data bss dec hex filename
2787425 78888 570048 3436361 346f49 slapd
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
2826 hyc 20 0 8442040 1.8g 222972 S 17.5 3.1 744:17.91 gnome-shell
1122733 hyc 20 0 5834092 2.1g 211008 S 12.3 3.5 1184:04 seamonkey
2603 hyc 20 0 3072140 32480 24128 S 11.6 0.1 2597:09 pulseaudio
1124256 hyc 20 0 3026040 391680 107860 S 9.6 0.6 638:12.95 Isolated Web Co
1123672 hyc 20 0 13.2g 697540 318016 S 9.3 1.1 611:31.12 firefox
Still pretty lightweight, seems to me. As for pain to manage, that hasn't really been true since the introduction of LMDB in 2011. BerkeleyDB was a pain to manage, before that, but nowadays It Just Runs.
I wonder how terrible DAP may be if LDAP is "lightweight".