There is a W3C presentation that helps frame the historical reasons for HTML5. The slides are available in plain text  and Technicolor. At the risk of confusing the facts,* I’ll try to explain why this is important and what is important about it.
For over a decade, XHTML1/1.1, a successor to HTML4, has been the most current version of HTML. XHTML1/1.1 leveraged the strengths of XML to create well-formed Web pages. These pages could be validated against a schema to test for compliance to a standard. Perhaps most importantly, it helped fix the issue of cross-browser incompatibility.
XHTML2 was going to be the successor to XHTML1/1.1; as the name suggests. However, this ended up not being the case. HTML5 is. Here’s why: XHTML2 was actually a different language; a new abstract approach to HTML. In at least the immediate future, making Web pages would’ve been made more difficult. It was a departure from the trajectory of many HTML traditions:
- IMG elements were being phased out in favor of OBJECT elements.
- The anchors, A elments, were being phased out because, “all elements may now play the role of a hyperlink.“