A successful website requries a large amout of maintenance. I can maintain any site using mainstream tools (PHP, wordpress, Drupal, Joomla, Magento, etc).
Quite often a popular website becomes popular and requires a greater work force than the orinial developer requires. Hirining a freelance developer allows you to just get the work you need.
Large web development firms have high overhead fees for their maintenace costs, if you have a small site we could reduce your monthly fee, for the same work.