Board Thread:Show-Related Questions and Answers/@comment-25491743-20150817210639/@comment-25375217-20151226232212

Just because it could hold Rumple doesn't mean that it can hold the Darkness itself. From what Rumple said, the Darkness was gaining more and more power over him. So back in Season 3a, the Darkness wasn't as in control, so it could easily be said it wasn't as powerful either.

That could explain it, too - if the Hat and the Box are about equal in power (as is implied), but the Hat was designed specifically for Darkness removal but failed, then the Box probably couldn't do it either.