There is also the unfortunate truth that there are only 9 (or so) base story ideas in existence. There are books written on the fact that there is no such thing as originality because its all been done before, and half of it during the years people always think Humanity was uncreative and "primitive" (I use primitive loosely yet pointedly), I.E. early nineteen hundreds and earlier. Most science fiction ideas were written during the early nineteen hundreds, a few or more during the eighteen hundreds.
So, with the knowledge that nothing is original, what are people to do? They have no choice but to choose something that has been written to death. Sure, some ideas have been used less, but those ideas do tend to be used less for a reason - they are disliked, looked down upon, seen as taboo, hated, and etc etc. The only thing people can do is change things up as much as possible, which isn't much unfortunately. Really the only "cure" to an overused plot is a writer who can capture the readers imagination.
I think, also, a big reason for things like the elf x orc plot is because people associate with humanoids better than other more extreme or exotic things. Especially if the person is less sexually secure. I have branched out a bit thanks to being more comfortable with my writing and ability to portray things and with my sexuality (online anyways). I've always been open to a lot of fantasy/sci-fi/strange/unusual things, so mostly for me its been more of a growth as a writer that has seen me expanding here and there. For others, I am sure its based solely on what their sexuality allows.
Then there is the annoyingly intrusive... society. Vampires in modern society are like a glorious plague. They can, at the same time, be so deviously delightful to see/read about/play as, yet also like as everyone knows -coughs- twinkling vampires -coughs-, can be so damned dreadful that it makes one want to stake the vampire mythos and toss it out into the sun to die a horrible death. My point, is society latches onto things, and people get fascinated simply because its what society tells them to like.
Also, keep in mind, some people intentionally "want" to play certain things because they know it will attract potential partners to RP with. I certainly have entertained the thought of creating some plot ideas merely to attract "anyone" to RP with because I feel empty and wanting to fill it with an RP. I
As for the frustration, I know how you feel. I see way to many ideas I consider to be mundane and as populated as Australia's mouse plague that happens every few years. I do wish people would branch out more, but for the most part its a selfish and unrealistic expectation. Not everyone wants to nor is capable of doing so.