What could cause a great artist to just quit for no apparent reason?

If you need inspiration, why not take a look at the work of street artists? These dudes haven’t let having no cash get in the way of their creativity, often resorting to using anything that will make a mark. So that large canvas is too expensive to buy? Why not take a leaf out of Charming Baker’s book? He left school at 16 and did various manual jobs before going back to college