This is an emotional issue. For many Hawaiians, the mountain top is sacred and connected to their identity. But astronomy is also one of biggest economies of the island. In fact, Mauna Kea has been picked to host the Thirty Meter Telescope (TMT) - the largest in the world (though Europeans are planning their own comparable one in the high desert of Chile) - and is expected to bring in substantial capital to Hawaii. Furthermore, it ensures the prominence of Mauna Kea as a premier place for astronomy for at least the next couple of decades.
The vote in favor of the management plan was unanimous. However, do check out these three short videos (about 5 minutes each) of the testimonies before the vote. These will give you not only an idea of the issues involved, but also a glimpse of the raw emotions involved - both in favor of and against the observatories. Especially, listen to the person that starts testifying in the middle video, starting about 3min and 30 seconds in. He encapsulates the complexity perfectly: the silver rainbow he saw recently on Mauna Kea was not pure silver anymore - but "it was still there".
I think the management plan is a good effort and addresses many of the past concerns about cultural and environmental issues. I hope astronomers genuinely appreciate the complexity of issues on the mountain. This is definitely a case of overlapping magesteria - and not NOMA. The least we (astronomers) can do is acknowledge it.
I will keep you posted about the status. You can read about the BLNR decision in Star Bulletin here.