Well there's a few arguements for that one:
1) Mainly because whoever ported it has a choice in what they choose to work on and/or could be something they are comfortable with doing and recompiling something that has already been written is ALOT less work.
2) regardless of age a good game is always a good game.
3) out of the vast ocean of individuals out there remotely interested in this type of thing only a few in comparison have experience in 1) the same field 2) share the same directives and preferences and 3) work comfortably in a team.
You have a good point but quite a few people can do little but port. It all needs to be organised and it all needs to be collectively accepted and pursued equally... not an easy thing to accomplish...?
This certainly isn't a definitive explanation and infact it might be quite choppy LOL, I'm a little tired and not even sure why I replied ;-)
