Did you notice any empty shells right before they died? When they molt they are much more vulnerable. Do you know if the ghost shrimp were young or old? I had an old white one (probably sick) that only lived a few weeks before he died during a molt. The older ones are obviously larger and they do not spend as much time hiding and can harass fish.
Not to sound condescending but did you recover bodies? The ghost shrimp will hide often and very effectively, my shrimp managed to hide with just gravel in the tank. When they die they turn very pink and look kind of like little worms. One good way I learned to lure them is with bloodworms, they will definitely come out of hiding for a bloodworm and usually perch up high since the fish scour the bottom, puts them right out in the open. If they are alive they also eat waste and detritus so they could be keeping hidden because they don't have to come out for food, if they are young maybe nervous with all the guppies or can't compete for food.
As riseabovethesun said they are sold as feeders by a lot of fish stores so they are kept in bad conditions, the only upside to that is they are very cheap.