We changed for a number of reasons, the first was that the previous CMS (content management software) was DotNetNuke, a Microsoft .NET based CMS system. The problem with .NET and DNN, were numerous, firstly the pages created by DNN were about twice the size of Wordpress files, but I had a hunch that the resources that Google dedicates to small websites is limited.
Secondly, the costs associated with operating .NET based site are expensive, both in terms of hosting costs and third party addins. I have reduced the costs of hosting the website and its associated databases to 10% of the previous cost.
Thirdly, the blog was not being read by search engines and other sites like LinkedIn, Feedburner etc. The new blogs via Wordpress are picked up by search engines in hours.
Wordpress is fantastic - its free, there are thousands of themes & plugins so that it can accomplish almost anything you want it to do. Plus Google loves it.
If you are thinking about a new website, you should get Wordpress.