Because our stylesheet expects its tags in upper case. At the moment, we automatically convert all tags to upper case while processing, but we may well have to change the way we process the GuideML such that it would be impractical to do this any more, so we insist that all tags are in upper case. If/when we change the processing method, the more articles which are fully compliant, the fewer problems we would have in switching over.

Because of the processing we currently do, we do accept lowercase tags, so nothing will break if you use lowercase, but a move to all lowercase as a policy decision would (at some stage) require a complete rewrite of all our stylesheets, and a massive job of processing all the articles in the database. Sticking to uppercase means we can keep the stylesheets as they are, and a much smaller job of processing articles if we move to a system that can't accept lowercase at all.