Viruses are only one potential cause of cancer, and there are several known tumorigenic viruses (HPV, adenovirus, EBV, polyomavirus, etc.). There are still researchers that believe that all remaining cancers are caused by yet undiscovered viruses (the "viral hypothesis"), but these researchers are becoming scarce.

It seems more likely that cancer can also be a disease of aging and degeneration. As we get older, things break. Cellular division is liable to be one of those things. There are identified mutations that dramatically predispose people to cancer, because one of the genes controlling cellular division is already broken (the "Knudson hypothesis").

Some researchers do prefer to use the viral cancers as a model for studying the others, because they luckily have a singularly identifiable cause, and therefore seem more likely to point to the causative mutations behind cancer as a general process. This is the premise for a paper that I helped author: http://www.nature.com/nature/journal/v487/n7408/full/nature1...

Nitpick: in the same vein, not all gastric ulcers are caused by H. pylori. Another common cause is chronic NSAID (e.g. ibuprofen) use.