After reading the RPS link[1] posted here[2], it seems to me that the
multiplayer-is-required aspect probably was bolted on towards the end of
development. While this doesn't preclude the possibility of fully preparing
their servers for the launch, it does hint that a lack of direction could have
led to confusion and/or a lack of planning for these issues. (Though this is
really just shooting in the dark because we can only go on what public
statements they make)
They've been making a lot of public statements about how the simulation works and their plans over the course of development (For example: [1]). Most of it nitty-gritty stuff has been in industry conferences and publications, but there is a fair amount of information out there. Probably not enough to absolutely pin down their mistakes, but there is some information.
[1] http://www.rockpapershotgun.com/2013/03/16/simcity-bosss-str... [2] https://news.ycombinator.com/item?id=5384314