We simply can't include every webcomic in the world. To avoid people using Wikipedia as a promotional vehicle we have to use inclusion guidelines to keep in stuff that deserves an article - highly popular (verifiably so) webcomics with a not so small run - and stuff that doesn't deserve an article - your average Joe site with 4 comics, a dead forum and no Google or Alexa rank because they don't have visitors.
Of course, Google and Alexa aren't everything, but if those can't be gauged to determine a comic's audience, we need the author to provide the information by other means. If the audience can't be verified it doesn't belong on Wikipedia.
We don't include unremarkable people, forums or ads for businesses. Webcomics should be no different.
How popular a comic needs to be and how many visitors it needs to attract is, of course, up for discussion.
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Example: We had several webcomics with bad Alexa ranking and dead forums. Someone complained that the forum recently moved. How are we supposed to know if they don't tell it or provide sources for the article. Any edit window asks people to cite sources, yet people fail to do so far too often...
--Mgm