There are a few reasons they are re-used over and over:
1) Inspiration. You'll often notice that major books will spark a surge in a type of tropes that will last for a fairly long period of time. This is because that major work likely inspired a major proportion of soon-to-be writers. Some very powerful works can inspire other powerful works so you can get a chain that continues the pattern. Orks, Elves etc.. would be one such situation whereby their continued use promotes their continued use.
2) Publisher choices. Publishers choose what gets published and even though we live in a time when self publishing is a viable alternative; there is still the influence of the publishing houses and what they choose. As such they might well cause patterns in what does and doesn't get chosen to be published and might well often follow patterns already established by best sellers. This in turn will influence writers keen to be traditionally published in what they aim to choose to put into their books.
3) It cuts down on world building/info dumping. If I say oak, elm, blacksmith, pub, castle etc... those terms evoke images in the mind of the average reader. If they don't then, as real world things, any reader can quickly get a rough idea of what they look like with a tiny bit of research (ergo very low effort). Thus they are terms which the writer can put into the story without having to flesh them out every time; or at least know that they don't have to flesh out every tiny detail because the reader will be able to fill in the gaps.
Orks and elves are like that; they are terms that already have a casual meaning and thus the writer can put an elf into their story confident that they don't so much have to explain what an elf is; but that all they have to do is show how their elves differ to other elves. By saying elf your average reader already knows it basically looks like a human; has pointy ears; has very fine and classic beautiful facial features; is likely "at one with nature" (or at least more so than man); is probably very very long lived etc...
Fantasy and sci-fi both have reasons to bloat their stories more than contemporary novels, with world building. So anything that helps cut down the bulk of world building is going to be a good thing as it helps avoid the reader getting lost in a sea of detail and losing the story.
4) Mythology. A lot of fantasy traces its roots to mythologies and since a large majority of western (english) fantasy is often based upon western mythologies (And very often upon Tolkiens interpretation of them); it stands to reason that we see the same mythological creatures reappearing over and over.
I would say that in more recent years there's been a subtle push toward including other mythologies. India and China are both popular choices as is Native American. However there are ripe mythologies in Africa, India and even if you look at Europe there's huge variations over the spread of countries. Often as note these elements are explored alongside traditional western mythologies - often as not when a book seeks to take itself into a series and starts to explore the greater world within the book and this the world setting.
Zombies/Nazis are slightly different in their reason for being almost abusively used; especially so in films/TV. This is because not only are they a very simplistic concept that everyone understands very quickly; but they are also natural enemies. They are instantly hated/feared/loathed etc.. and thus easy fodder to be shot, maimed, gutted, melted, abused etc... without risk of alienating the majority of the audience. They are thus a nice and safe evil to throw into films without worries. Hence why they get used and re-used over and over again.
Of course inspiration and other elements also come into play as well; the vast numbers of wartime films produced in the war and post-war period made the Nazis a huge inspiration as the great-enemy for many writers and directors and thus spawns its own continued re-use.
