I suppose this is because the original developers knew exactly what hardware this would run on and the achievable framerate on the PS2 and so took lots of short-cuts in the code?
Edit: Incredible project by the way.
I hate to say it, but fixed frame rates these days seem very 'lazy'? Obviously depending on use case and hardware.
If you know the game is only going to run on certain hardware, and you make the most money during the launch period of the game, it doesn't make sense to spend time, effort and money on something niche for the future.