God allows billions of people to learn from the book of Mormon, the Koran, and many books of Eastern religions that claim to hold His truth or the truth about Him. There are many people within these religions who would love the truth and earnestly seek it, even if they are presently unable to see it.
Perhaps the bigger issue here is that Protestants are attempting to being reliant entirely on a book that is NOT 100% preserved in its original form, that may contain errors due to omissions, additions, alterations and so forth. The book is not supposed to be a substitute for personal intimacy with God.
Really, just think about it: until a few hundred years ago copies of scripture were not even widely available, and even when they were available they were usually written in a language most of the people did not speak, nor read, and too, most of them could not read ANY language, much less Latin, Greek, or other language common for biblical copies.
Sola scriptura only works when there is widespread literacy and scriptural access.