No! I am not trying to count the reasons. Dear reader, if you came here to find the answer to that, I'm sorry.I'm in the same boat as you are. I just do not understand why?

1. It has no separate DB access layer; it supports only the php & MySQL combination; that too, limited versions (the latest stable version of Joomla doesn't support MySQL 5.x).
2. It is not SEO (Search Engine Optimized; as it's otherwise known to normal people), although some may argue that's not true, but it's not easy at all. And if you have a site with numerous custom components, just forget it.
3. It is severely under-documented. I remember a little above a year ago, we would find sections and sections of the official Mambo documentation with only "TODO". It was utterly agonizing & frustrating experience. Although significant improvements have been made in this area since the Joomla avatar, it doesn't fare as great in terms of structure and content as that of Typo3 & Drupal.
4. But the biggest problem I have faced as a developer working on Mambo/Joomla is the lack of separate data/presentation layers (separate Model & View component/layer). And to add to that there are lots of hardcoded presentation elements in the core code, and seems like it's hard severing the ties to the Mambo past, despite great efforts to modernize the code since Joomla first came out more than a year ago. There have been quite a number of occasions when this fact has bitten me in the butt.

I have led the development of a fairly large website serving thousands of users using Mambo and not Joomla (4.5.x, the development began months before the Joomla team left Miro International for good). Since then I've also had deployed 2 more sites (not nearly as large as the first Mambo project), call it utilizing resources… why let the newly acquired skill of my team mates go to waste. And had it not been the clients' insistence I'm sure I wouldn't have used Mambo.

Seems Joomla is having a makeover starting with version 1.5 with the biggest change in the form of the separation of the logic & presentation layer. But does a minor version increment is justified with such a huge behavioral change?