I think that some of the parks may be discouraging some volunteers by requiring longer commitments and more hours. I was turned down as campground host at a Georgia state park because I only wanted to give them one month, and their policy was that volunteers must work at least 2 months, but not more than 6. I was turned down, even though they had an unfilled position. I understand that they don't want to be constantly training new volunteers, but how much time does it take to train one.
My objective was to 1) contribute something to our parks, and 2) to spend some quality time in a given location. I do think my time is more valuable that the benefit received, but every thing is not about dollars and cents.
By requiring longer commitments with more hours, I think the parks are limiting themselves to volunteers who NEED a camp site, rather than volunteers who want to make a contribution.