XML-RPC is enabled by default since WordPress 3.5 so either a plugin or a setting is causing to disable it.
What plugin do you use for security and caching? Search for the XML-RPC setting and enable it.
Why is this disabled by your security (or cache) plugin? To prevent attacks on the xml-rpc.php file.
By enabling XML-RPC you can edit and post content but you are also more vulnerable to Brute Force attacks.