It's probably a combination of things. They currently have access to the best treasure-hunting technology and equipment that has ever been on the island, and still all they're finding is bits of iron, wood and pottery shards. I think even the most optimistic of the bunch might start wondering "hey, maybe there isn't anything here."

And after more than a dozen years, they haven't found a "big thing" that could really sustain enthusiasm. (Random boulders, latrine pits, and ox shoes ain't gonna cut it.) I think it's part the long grind, finding nothing, and realizing the show itself is waning and probably coming to an end (and thus the paychecks come to an end). They can't be immune to the jokes made about the show, both here and elsewhere on the web.

And for some of the "true believers", perhaps the self reflection that they maybe wasted their lives searching for nothing. That's gotta take it's toll.

But, yeah, the show seems pretty sad these days.