1. Expecting current political and economic structures etc to remain for thousands of years is silly, so we do need a more permanent solution to a long-term problem.

2. Keeping lots of nuclear waste in spent fuel pools just increases the risk picture unnecessarily.

Mostly corrosion. The half lives involved, combined with the great expense of safely adjusting these things after they're put to rest, means that you would really like something really dry and remote. Fear of contaminating ground water is incredibly sensible to me, at least. It'd be a hell of a thing to get Yucca Mountain un-cancelled...