I've always seen a question given to those who want to write a new commercial role-playing game: "What does your game offer that [system name with similar concept] doesn't?" It's a pertinent question that anyone hoping to sell RPGs should try to answer. But what if you don't plan to sell anything? What if you just want to make something?
The idea of targeting a "niche" is also complicated by modern filesharing: Why would a person look at your homebrew Lovecraftian horror system when he can just get the latest Call of Cthulhu PDF with a little googling?
Are things like darkshire's list of free RPGs or 1000 monkeys, 1000 typewriters relevant anymore? Before sites like those offered free alternatives to the costlier products from big publishers but now filesharing (or piracy, take your pick) is so easy and reliable.
I guess I'm motivated by egotism: I want to write new games but I also want people to at least read them. Putting effort into something no one or almost no one will read seems like a waste. But then again, I've started work on so many systems over the years only to end up abandoning them, so does it really matter if anyone reads my rules?