Because, simply, folks that are predisposed to look upon the GLBT community as "other" already have serious negative pre-conceived notions about what we do when no one is looking. Parading about in drag, leather and thongs may be lots of fun for some, but it reinforces negative stereotypes and is not representative of our community in total.
Mardi Gras, OTOH, only reinforces the notion that "Geez, people get crazy when they're drunk, don't they?"
you are correct, of coarse, but i've always admired people that don't need to get drunk to have a good time and i've always had a good time at events where there was a majority representation of lgbt(did i fuck that up?) persons.
(not that theres anything wrong with being drunk)