Mark Story goes over a few reasons of why CakePHP 3.0 breaks compatibility in this blog post. If you are working with CakePHP or involved in any large system that lives forever, you should read those in detail. Otherwise, here is an overview:
- PHP has changed
- Ideas that didn’t work out so well
- Outdated implementations
- Improve consistency
Also, if you are working with CakePHP, you should attend the CakeFest 2014 event next week in Madrid, Spain.