Well a flopped game doesn't mean the game was bad. 1.3 million sold is not good for FF franchise. Plus as other mentioned the accessories and dealing with co-op. At the time people wanted a FF game like what was on PS/PS2 and people were pretty disappointed with what they got (Im basing this on my age group at the time, 16+. Reviews and players complained about weak story compared to other FF titles as well. Gamecube owners felt like they got a hand me down to just shut us up about FF on Nintendo. It was made by a shell company with the agreement development would not take away/affect PS2 FF game. The reason SE jumped on GC for extra $$ was due to financial failures of the FF CG movie and the competition between Nintendo and Sony faded as xbox came into the competition.

The game did bad probably due to timing, co-op requirements, etc. And like I said people were, at the time, expecting something close to FF on Sonys consoles.

I think it will get a 2nd wind with the re-release since all those early 2000s expectations are gone.