The first reason would be because hiring a web developer or designer is going to consume far more time than getting website templates. Think about it, you have to explain to the web developer exactly what you want. You have to go over it with them again and again and again. You will have to monitor what is being created and make adjustments during the project. Once the project is finished it is probably going to be behind schedule, and adjustments will need to be made that might cost you more money and more time.
Joomla and Mambo CMS have been developed by the same professionals of web design industry. So, they possess a lot of common features. For example similar code, system requirements, etc. At the same time new releases of Joomla will differ from Mambo. Having compared Joomla and Mambo CMS‘s you can notice that Joomla has more options, better security issues, regular updates, additional Built-in-Applications, etc.