Why the Royal Navy retained so many older battleships

Why did the Royal Navy keep so many older ironclads and pre-dreadnoughts until Fisher's reforms?

Fisher did not remove them all, and the Royal Navy retained a very large pre-dreadnought inventory that still served during the First World War.

Fisher did not immediately eliminate the older fleet, and many pre-dreadnoughts remained available into the First World War. Their sheer number kept them part of British naval strength even after newer designs eclipsed them.