We already had so much of it, the renewable targets would have been too easy? Most dams use huge amounts of concrete or earth moving, and there’s no carbon-free way to do that work at this time? Policy makers didn’t want to incentivize any dam construction?

These are guesses. It’s probably in some kind of records. Sometimes laws even explain the rationale in the beginning with a bunch of “whereas” statements. But I’m too lazy to look it up right now.