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Because of it easiest to use CMS, and have free plugins for any you need.

Because WordPress provides you built-in plug-ins which work by their own instead of what you are working manually. it is also easy to use rather than PHP, so is that everyone is using WordPress even thou it is unsafer than PHP

Everybody is using Wordpress because it provides easy to use interface and ready to use tools. One can use the desired themes along with the plugins for there site. Apart from that, one can easily customise the layout of the site.