This is a very popular question and has been answered several times:

https://www.reddit.com/r/AskHistorians/comments/2sq8wk/why_did_the_ancient_mesopotamians_use_chariots_in/

https://www.reddit.com/r/AskHistorians/comments/2xocwx/why_was_the_use_of_chariots_in_wars_popular_in/