The short answer is that LDAP provides more advantages over
DAP than just running DAP directly over TCP can gain, mainly
relating to the data encoding models (strings vs BER), and
general protocol complexity (LDAP leaves out the bells and
whistles). Jeff also makes some good points about diversity,
etc.
While LDAP gets many good things from X.500, being even
more of a slave to the ISO standards process is not a good
thing in the Internet environment. It's good to take the
basic stuff (information model, naming, etc.) that is stable,
but be free to evolve the other parts independent of ISO.
No, you're not the first to suggest this. Originally, we
thought there might be several "lightweight" protocols,
including one like what you describe. But, things evolved,
LDAP has the momentum, has been embraced by a wider community
precisely because it is not a complete slave to X.500 (reason
#23412), and I don't think a lot of people see a need for
the other approach now. -- Tim