I would hazard to guess that it has something to do with life expectancies: according the HSUS, the life expectancy for an outdoor cat is 3 years old; the expected lifespan for an indoor cat is 15 yrs. Hence -- the longer your cat lives, the more likely it is to develop health problems.
As well, cats that go outdoors also eat wildlife, which is undoubtably closer to the diet cats evolved to. Eating small animals and birds probably helps regulate the urinary ph level. Considering a lot of cats are fed gross foods such as Cat Chow and Meow Mix, it isn't any wonder they are getting sick.
Keeping a close eye on your cat's litterbox behaviour will help alert you to signs of FLUTD (feline lower urinary tract disease). My late Gil got it in 1995 -- he cried pitifully while digging in the litterbox, and he had an accident on the chair he was lying on. I rushed him right away to the vet's: because of the early intervention, it was easier (and cheaper) to treat.
Males are more susceptible to it because their urethra has an "hourglass" shape to it; females urethras are wider. However, females seem to be more prone to bladder infections caused by bacteria -- the bigger the hole, the more that can get in!
FAQ 
~Sue~ 
~Pablo 
~Lucia 
Lavander 
: Together in Singapore 
