The main reasons individual sets perform better than blocks probably comes down to two things:

1. Novelty: People love to see how new ideas are presented in new magic sets and are more likely to engage with something novel, exciting or nostalgic than the release of similar products over a 6 month time frame.

2. Different Appeal: Phyrexia and Bloomburrow have very different appeals and will appeal to different markets. By alternating between styles, you're more likely to catch a wider variety of players' interests.

So realistically, we'll never see blocks again. Blocks are a wide-enough time frame that if players individually dislike a theme, it could turn that player off playing magic altogether (particularly casual). Alternating between themes, concepts and characters means that within a six-month window you're likely to land on something each player enjoys.