If you read Rick Parry's reply to some questions put to him on Offal site, it would cost the same to redevelop as it would be to build a new 55,000 seater (which has the option of further increases in capacity).
On the current site, 52,000 or so is the maximum that it could be extended to ever. For the same price you are going to get a brand new stadium, etc with additional benefits. Anyway, read it, it's on the official site site and answers a few of the queries why the club has made this decision.