When it comes to veterinary care, cats seem to get less of it than dogs. According to a study published by the American Association of Feline Practitioners (AAFP), more than half of cat owners haven't taken their cats the vet in the last year. Besides that, older cats don't see the vet as often as younger cats, and vets believe in annual checkups for cats. Half as many cats get annual checkups as dogs.
Dr. Patty Kuhly wrote on Vetstreet that there are a number of contributing factors to this, including the recession, which contributes to being unable to afford the rising cost of care, the availability of information on the Internet, feline behavior (they don't like going to the vet), and an odd perception that regular checkups aren't necessary for cats.
However, Dr. Kuhly doesn't believe that these contributing factors tell the whole story. They can easily apply to dogs as well. She believes that the factors at play have more to do with the fact that cats are independent creatures, and are also very good at hiding the fact that they don't feel well. Also, while there are lots of pets out there that hate going to the vet, it seems to be an almost universal problem with cats. They also don't like to travel, which can deter cat owners from trying to take them anywhere out of fear of getting scratched or bitten.
Another issue is the way in which we get our pets. On a nearly universal basis, people adopt dogs intentionally, but often get cats accidentally (the stray that won't leave your yard, for instance). So there might be a bit of a perception that the dog is more valuable to the family than the cat, because the cat just wandered into their lives of its own accord, whereas the dog was deliberately chosen.
The problems with this mean that cats may not get the timely care they need either when they're sick, or necessary preventive care that can come with annual visits. An article by Dr. Joanne Intile, on PetMD, discusses the fact that cats do get cancer as often as dogs, and the cancers that vets treat most often in dogs are the same as in cats. However, she brings up an issue not addressed by Dr. Kuhly, which is that, in general, there is much less information out there about cats than dogs.
Dr. Intile, like Dr. Kuhly, also touches on how cats hide their symptoms. By the time you notice a problem, they're often seriously ill, so the outcome of treatment and prognosis is worse. Dogs also have a natural instinct to hide their illness, but because dogs tend to be more active than cats, it's easier to notice when something's wrong early, and get prompt treatment.
One thing you can do, if you think your cat isn't getting the attention it deserves from your current vet, is find a cat-friendly or cats-only clinic. This website contains information on the Cat Friendly Practice Program, and this is a list of cats-only clinics for each state and Canada.