If they did it that way, with some players the match would never have the Rumble feel. The ring would always be clear, and then you could nothing but pose for 1:45 and build up 6 Smackdowns.
That's exactly why I don't mind the current Ironman/Rumble setup. I mean, I agree with giving the gamer the option to SET the time limits for these matches, but the current settings that they're at are just plain 'realistic' when it comes to a videogame. I, personally, would not sit there and play an ironman match for 60 minutes straight. Holy crap, there'd be no point, seeing as with the current ironman, I already have about 5 falls after 5 minutes.