It shouldn't be surprising. At some point, you (mostly) exhaust what you can do with the available stats. Then you wait until data collection improves and begin again. With MLBAM rolled out next year to all 30 teams, we should see an explosion in the next year or two.
Also, as each new stat improves on previous ones, the amount of improvement is generally smaller. At some point, people (at least the public) don't care about the difference in the small amount of improvement, especially if it also comes with an extremely complicated formula.