I think part of the reason why melanomas get diagnosed late is because doctors don't want to do a biopsy unless it's necessary. Of course late diagnosis also has to do with the fact that people don't pay attention to their toes very much and toenail melanoma is not all that common.
But the other reason is that doctors don't want to do a biopsy because a biopsy can actually spread the melanoma. I had a very hard time convincing my doctor to do a biopsy. He finally removed the spot entirely and tested it. Thankfully, it turned out to be non-cancerous.