Why wordpress is very popular?

WordPress is a very popular open source script used as publishing platform.

1)Wide applicability:
It can be used across many industries.
2) Templating system:
Templating system can help to change template of wordpress and hence you can have any look which is similar to your site / your needs.
3) Open Source
As its open source which is written with PHP-MySQL, a very popular web development tools. Using script one can edit themes to have advanced effects / facilities.
4) Search Engine Optimization:
The url management is search engine friendly and often permalinks are provided.
It helps in getting your site listed in Search engines.
5) Huge plugin architecture:
Wordpress has very huge plugin architecture. You can add many useful facilties which can be added as a sort of plugins.
6) Integrated Stats:
Wordpress gives you up to date stats on how many people are visiting you site / blog, where are they coming from, popular posts seen by the people, search engine terms used to search ur blog etc. In short it presents good analysis which one can use to take decisions.