Recently I've been doing some research on pros and cons of using Internet mediums such as "Square-space" vs Wordpress for creating web sites... Wordpress is the most popular medium for blogs and web creation currently.

Considering long term cost as well as usability / flexibility / support / plug ins etc as well as search engine optimization ; rank/ self hosted servers etc .
What makes Wordpress the number 1 choice in online marketing / blogging & business creation over other mediums to beginners and vets alike ??

Any info across the board is appreciated!!