Actually, it's covered in the site rules:
You may vote however you wish on any entry, provided that this done as an honest expression of opinion on the article and not the author or other outside factors, and not as an attempt to artificially boost/reduce an article’s position as part of a “group strike”. A “group strike” is defined as the mass voting of an article based on things other than the article’s merit, such as a personal opinion about the author, outside pressure, or other things with no bearing on the entry itself.
Consider that page status on the wiki is based on ratings derived from these votes, and that voting criteria is totally up to the holder of the individual vote in question. Shitty pages are the ones that get enough downvotes to be deleted.
So yeh. What makes an article shit? Totally subjective. That said, obvious things like incoherence, bad writing, misapplied humor, lots of typos, and other blatant writing problems are garaunteed to not go over well. That is why we have guides geared toward teaching users not to commit those crimes.
Thing is, what I'm seeing in your question is less "what makes an SCP shitty?", which is the question the guides exist to answer, and more "why do people downvote?", which, being entirely subjective, is not a thing staff can answer, given that we're authors, community members, and editors, not mind readers.
Now, if you were hoping for us to impose a standard set of voting criteria and expect all users to adhere to it, please excuse me while Iahahahahahaaaaahahahahahahahahahaha
