The difference is that Spring Training is mandatory for players, and it's actually part of MLB and its teams, it's important they get ready for the season. The WBC is completely voluntary and it is pretty much an exhibition. Players are also on strict training programs and starting to do stuff early can be detrimental to getting prepared for the season. A lot of the pitchers that were in the WBC last time had hard times staying healthy/effective throughout the season following it. Players get paid to play for their MLB teams and take part in events/games with their team, not to play in some meaningless exhibition. It would be different if players were making only thousands of dollars to play MLB, but when players are making $10+ million a year there is no need for them to go out and risk injury playing in an event that has no meaning, when they are being paid by a organization to play for their team. I think fan interest dictates whether an event has meaning or not. And the WBC has a lot of fan interest, way more than expected back in 2006.