Specifically because Glacier Peak * is in a wilderness area, and it could take as many as 20 helicopter flights into the wilderness area to install permanent seismic monitors.

It's the same group that sued to have a historic fire lookout (Green Mountain) removed, because the Forest Service used a helicopter to move supplies to restore it. I suppose one could make a case that preservation of a historic site in a wilderness area was not adequate cause to justify the use of helicopters, but they insisted for years after the work was completed that the only acceptable remedy was removal of the lookout. The fiasco didn't end until Congress passed an amendment to the Wilderness Act allowing the lookout to stay.

Every time I see Wilderness Watch come up in the news, I find myself debating whether it's just another law firm that likes to cash in on reimbursement of legal fees masquerading as a conservation group, or if a bunch of obsessive-compulsive types who couldn't find an HOA to ruin simply banded together with misconstruing the intent of the Wilderness Act as their common goal.

* Glacier Peak is considered by the USGS to be the 15th most hazardous volcano in the US, out of the 161 volcanoes they assessed. Its ranking in the "very high threat" category has made the fact of its very limited monitoring a major concern:

https://pubs.usgs.gov/sir/2018/5140/sir20185140.pdf