Why did the Protestants stop using the lost books of the bible?

your question assumes the books were dropped. The church that existed long before the catholic church was established, already had a set number of books.

that church is what is known as the protestant church today.

long before the catholic Church came along, the books that were considered holy books were well established. the catholic church came alone and tried to add, all be it probably good books, to the books that would become known as The Bible. the problem is they do not meet the standards required to be considered holy books. they are just regular books just like any other book out there. There are very rigorous test which any book that would become part of the bible would have to go through to be included. and those books do not meet those standards.