When Milo opened Coolsville, it was an immediate hit among the kids of Springfield, for several reasons. Compared to The Android's Dungeon, Coolsville was larger and carried a wider variety of comics and other memorabilia. However, the biggest difference between the stores (and the biggest reason for Coolsville's instant popularity) was the contrasting attitudes of the owners. Where Comic Book Guy treated his customers (kids in particular) with indifference and contempt, knowing he could get away with it since The Android's Dungeon was the only comic book store in Springfield, Milo treated the kids with respect and was genuinely interested in their opinions about comic books.
Very quickly, the kids all stopped buying their comics at The Android's Dungeon and came to Coolsville instead. The Android's Dungeon was then forced to close due to lack of customers.
Despite the popularity of Coolsville Comics & Toys, it has only been featured once and never heard from again, while The Android's Dungeon (and Comic Book Guy) remain ongoing through the rest of the series, as not resetting everything would've required Jack Black to become a semi-regular member of the cast of The Simpsons.