If they just made Castrum give EXP people would still eagerly be doing it.
As it stands, if you die (and you probably will at least once or twice) you lose mettle, and running the entirety of Castrum doesn't award any mettle in kind, so I see a lot of people just not wanting to do it because of that. Just give a nice mettle bonus at the end, and boom, it'd help a lot.