I wonder - was there really no other people they could have given it to? The detection of gravitational waves was split between a theorist, experimentalist and a person who had a big hand in shepherding the project along. Could not the same have been done here?

I would imagine the problem is that there are too many of them.

For better or worse the prize can only go to 3 people. Over the years there are generally many dozens of people who make absolutely critical contributions to these kinds of experiments. In this case, though, the same guy was listed as the PI of the UW Madison group, and Madison is very clearly "the" operator of the project.

Halzen is by any measure an awesome physicist, but he's also a good "fit" for the Nobel because of this unique situation.