According to the Wikipedia definition of XML
Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. The W3C's XML 1.0 Specification and several other related specifications—all of them free open standards—define XML.
Basically XML is a set of rules to make sure every document has the same standard. If you have a couple of hours to kill and wants to get bored to dead, you can read the specification here.
Back to your question, it was a design decision to have only one root element in a document.
One of the main advantage is to allows to build a tree structure. It's easier to parse a logical tree with a predefined structure then a collection of node of different types floating around.
With a predefined structure, it's also easier to traverse the document and find the information you're looking for.
