There is no good reason. But now it's scary to change it, since there could
be places where we assume that usernames can not begin with a lowercase
letter. There is task
https://phabricator.wikimedia.org/T28396 about this.
Although folks are complaining about it, it seems that no one has proposed
a patch. (As long as the default is still to user capitalized usernames, I
don't see why we couldn't have an option to allow lowercase ones.)
For example, one case where we assume usernames can't be lowercase is in
some "shortcut" URLs for special pages. URLs like
https://en.wikipedia.org/wiki/Special:Contributions/Matma_Rex and
https://en.wikipedia.org/wiki/Special:Log/Matma_Rex will display the
edits/logs of an account, but URLs like
https://en.wikipedia.org/wiki/Special:Contributions/newbies and
https://en.wikipedia.org/wiki/Special:Log/block have special meaning (and
the edits/logs of a user named "newbies" or "block" couldn't be
accessed
like this). In this case it's harmless (you could still access them using
the form), but there might be assumptions like this in more important
places.
--
Matma Rex