Many developers develop WordPress based websites for their clients as this CMS is used worldwide and has many features and functionality which are better and makes this CMS the most desirable of them all. Also the learning curve is very less and to add a functionality all the developer needs to do is find a similar working plugin and configure it for their clients. Similarly many developers do not wish to show to their clients as to how they created their websites and do not wish to share the admin full fledged features like plugins etc.