You seem to be making two points here:
- It's easy to figure out which items will sell better than others (using modern market-research techniques)
- WotC has some sort of agenda other than giving their customers what they want
I don't grant either premise. The vagaries of what makes one item "click" with the public while another falls flat are exceptionally complex, especially when dealing with aesthetic issues. Besides, even if market research could overcome sales inefficiency, it isn't free. You're reducing one cost by adding another. Merric's law is upheld.
As for the second point--well, I don't really know how to address it. WotC has struggled with Merric's law just like everyone else; if they haven't delivered exactly what the public wants in one regard, it's only because they're trying to deliver in some other regard. I don't think there's anything more sinister than that!