Easy to program for. But as systems evolve they get more complex. Then a new generation comes along, forgets everything we once knew, and all of a sudden things are simple again. It's the breathwork of technology. I never thought of it that way.
Simplicity can also come from experience and overview.
If you look at XML, it is SGML with the experience of what can be left out. On the other hand, for people who don't know SGML, XML can look strange and complex. XML might have been simpler if the designers would have started from scratch. SGML people complain that the W3C doesn't look enough at what they've already done, but perhaps they looked too much.