Default namespace for attributes?

["Mukul Gandhi" <gandhi.mukul@gmail.com>]

The XML Namespaces Recommendation says that the default namespace
applies only to elements, and not to attribues
(http://www.w3.org/TR/REC-xml-names/#defaulting).

i.e. an unprefixed attribute name will always be in the null namespace.

I am curious to know why this is so?

Regards,
Mukul