Marco Arment wrote an astute article regarding the iPad 1’s lack of upgrade potential:
The iPad 1 was the first modern “tablet”, and as we saw (eventually) from its competitors, its $499 price point and excellent battery life were difficult to achieve in 2010 (and even in 2011). More RAM would have added to the component costs and decreased the battery life, potentially making it less appealing and jeopardizing its success, so Apple chose to keep it at only 256 MB.
Whether that was a good decision or not, it significantly shortened the iPad 1’s useful software life.