I thought that was interesting, too. My theory is that Big air is just one jump, not a series of jump, so they feel they don't have to be as forgiving for falling. In each run of slopestyle and half pipes, there are several hits you have to make, each one with a chance of a fall. With Big air, there's only one per run. Plus they want versatility, so they require the 2 jumps that count to be different. You can't do that if you throw away 2/3 runs. With slopestyle and halfpipe, you can show your versatility in just one run.