several years ago, to encourage people to come to the site also on weekends... since traffic on the site was slower during Sat-Sunday, and the number of ports was much lower.
With more people playing the game now, and with the implementation of changes (owning 100K shares, etc) HSX felt that the need to offer free trading was no longer needed.
Driving 'traffic' to the site, is what they are trying to do, and although have free trades on weekends helped, it also caused a lot of problems with day trading stocks, port swings in value, etc...
My guess is, HSX felt it was no longer needed as an incentive, so it was dropped... but that's just my take on it.