Atlassian Confluence team – Why We Removed the Wiki Markup Editor in Confluence 4.0:

“Wiki markup as a storage format hindered our ability to add new features, like merge table cells, that customers had been demanding. This is because Wiki Markup is a very limited subset of XHTML and because any new editor feature had to be built twice...once in the RTE and once in the Wiki Markup Editor. We also had a lot of bugs when toggling between the two edit modes. We knew for some time that we'd need to unify the dual-RTE and Wiki Markup Editors into one simple-yet-capable editing experience and store Confluence content in a more extensible storage format – i.e. XHTML.”