The issue with ipad sales is that they really made it TOO good from the get go and only improved it from there. People have no need or desire to upgrade. Their first purchased ipads still work amazing and given the cost why would you buy another until you need to?
I'd hardly say that the early iPads still work amazing. The original iPad was slow when it was released. Each model except for the iPad 3 was a noticeable improvement in performance.