Much like in the world of construction, renovation is a lot trickier than the initial construction. All the plumbing and electrical is already in place and you have to work around that.
Phoenix 1 was essentially “grown”. Each piece was built and tested little by little. Contrast this with “the Rebirth”, where everything is already inter-connected and it all has to converted at the same time, before even the first line of code can be truly tested.
One of the changes is to more loosely couple the code pieces where possible, so the pieces are more often connected through an intermediary. It doesn’t have a performance penalty, but does more easily allow pieces to be swapped in.