It would be a shame if there is a cache-killer on the loose in your area, but I really don't think the problem of dissapearing caches is unique to Denver. As to not maintaining caches, well, that's almost impossible. I, for one, now have caches in several States and it would be impossible for me to revisit my cache in Hawaii (although I'd really like to). It's up to the geocache community to do a bit of cache maintenance on caches we find, i.e. rehide them well, make sure we don't trash the cache area so that it draws undo attention, dry out a cache that got wet, etc.
I think maybe part of the problem is placing caches in rural and/or oft-visited areas. A cache placed in a popular park is more likely to be spotted and stollen by a non-geocacher than one placed in the desert south-west.
I did have a chche stolen from a small park in El Cajon, CA last year, which I replaced. I'm not sure if it was found by a passerby or what. I thought it was hidden rather well. What probably happened was that an on-looker spotted a geocacher logging the find and took the cache afterward. It's a lesson we all need to learn - be discrete when placing or logging a cache.