Why another CMS? Its a mixture of reasons, of which the main ones are:
I found out about Php around 5 years ago playing with other Open Source systems such as Joomla, Zen-Cart and the likes. Doing quite a lot of custom development in the form of Joomla modules and plug-ins at one of the Development companies I worked at, I found it rather hard to do some basic things. Even though these basic things were part of the Joomla core. As an example getting pagination working was not a simple matter of $this->paginate(), rather it was a good few pages of code, which seemed rather lame considering pagination was obviously happening in the core “modules”. Something that has all the power of the core functionality makes developing extensions much easier.
Something that most open source projects suffer from is a nasty divorce, leaving a lot of horrible legacy code and conventions. Joomla and Zen-Cart both have this problem with Joomla coming from Mambo and Zen-Cart coming from osCommerce. There is no doubt they were better off, but the scars remain. Starting from a clean slate with a clear goal makes for a better organized code base with less hacks.
The other big issue I had with them, is while Joomla is almost without doubt the best content management system around, its not cut out to do other things that a website normally needs. Sure there are plug-ins, but have you ever tried setting up a shopping cart with VirtueMart? Not much fun. Try doing some content in Zen-Cart, it can be done but far from enjoyable. I wanted something that was designed to be flexible from the ground up.
This was all a while back and I have not done much development work in them, but from the interaction I have had, not much seems to have changed. I stopped using Joomla just before 1.5 went stable.
Add this and a few other things like, tables, procedural code and general bloat and a new shiny content management system is born. Infinitas was developed on the CakePHP framework with a MIT licence so its yours, free to do what you like with it. Hope you enjoy it as much as I do :)