Because that's who buys the books.
Top rated comics aren't selling one million copies anymore. Kids aren't going up to newstands with all the quarters they saved up this week and buying an industries entire run of characters from Amazing Spider-Man to X-Men. Therefore it's not some group of uniformed, deluded masses who are going to the comic shops in droves to buy the latest Spider-Man, but might not if only they knew what real Spider-Man stories were suppose to be like.
The "sales don't mean squat" argument only really works if you can present some argument that the general public who consumes the good is largely uniformed. For example, as a Bodybuilder, I can tell you I am not a fan of MuscleTech products, they are just clicks and whistles. Practically snake oil. But, they are probably the top selling brand, outselling some real great supplements made by lesser known companies. Why? Because MuscleTech products pander to "da masses" by promising outrageous gains and presenting very flashy gimmicks and bright colors to sell their products.
Comics aren't really like that though. Most people who walk into a comic shop and fill out a subscription list, or pick up this or that issue every month have a general understanding of who the Green Lantern is, or what the X-Men are. They are probably familiar, at the very least from osmosis what the classic storylines are, who some of the great writers were and who some of the iconic artists are. These are the people buying the majority of the books every month. So why do sales matter? Because it means these devoted fans are invested enough in the current crop of stories to buy them. If they didn't like them, they wouldn't buy them, and we've seen that happen in the past.