Stores are going broke because no one is visiting them, or not enough people are visiting them.  I, along with my friends Butterfly_fish and Valerian70, ran our own content store after a bad experience with another venture that we'd been with.  For me, I wanted to test the waters, but I was actually too intimidated to upload either here or elsewhere.  My freestuff was doing great (most of which is gone, due to the original props/clothing no longer being available), so, we decided to open our own store.  Our host was Hostgator, we paid $8 a month for hosting.
The problem was, even with advertising here (when it was still allowed) and DAZ, we might hit about $5, on average, per month in sales.  Being that there were 3 of us, 2 with full-time jobs, and all of us with kids, it wasn't feasible for any of us to sit at the computer all day and make products, run the store, and answer any questions in the forums (which were the most active part of the site).  None of us wanted to churn out product after product and samey stuff all the time, so, we migrated to CP, where we might get more exposure.  
That kind of worked.  For a while, until their last change, and then we decided that it would be for the best to part ways with CP.  Due to database problems, server problems, and the like, we never re-opened the store at CGRealms.  I've moved my freebies, but my old products that clearanced out here aren't sold anymore.  I might end up putting them in freestuff, if I can find them again, but, that's beside the point.

The fact is, having your own, smaller, store will only work if you already have a huge customer base that will follow you to your own store.  On top of that, you have to spend time administering your site, making sure products are tested, making sure customers are getting what they paid for, and answering questions.  (Yes, we tested our products.  All 3 of us were testers at one point for either Renderosity or RDNA.)
The cost isn't just hosting.  It's time.  It's reputation.  It's advertising.  It's marketing.
If you're not good at marketing, your small store is going to tank.  If you don't have a ready-made customer base, your small store is going to struggle.
It takes more than paid-for hosting and the will to make a store work.  You NEED customers to visit and make purchases.  It needs to be worthwhile.  A small store might be able to get away with small-scale hosting like hostgator, but the larger stores don't use hosting like that.  DAZ's, I believe, is in-house.  IIRC, Renderosity uses an off-site, small-scale server farm that is close to the main office.  I don't know what RDNA uses, offhand.  Regardless, none of them would survive on a hostgator account ;)

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Knowledge is knowing a tomato is a fruit. Wisdom is not putting it into a fruit salad.