If what I've read is accurate, the various departments were already served by smaller systems that fit their specific needs.
The intention with Phoenix was to save money by consolidating them all into one. Oops.
At least part of the problem is that projects like this don't end up being awarded to those who are most capable of completing them successfully. Instead, they end up being awarded to companies and consultancies who are experts at shepherding proposals through the RFP process.
Maybe if they started with a subset and slowly built up..