Because I don't give into the geek fallacy that every person has to game together simply because they like rpgs. I am not going to run a game that I will not enjoy running whether it be flavor element or mechanics just as I don't expect players to play in a game that does not interest them. I offer what I offer and those interested will play . Those that are not don't (and free up a seat for someone else that does). To date, however, it has never come to this, but it come close once with one player who was initially reluctant to be restricted. By the end of campaign, the player was glad that he continued to play. Despite us being in someone else's campaign, he would call me asking when I could pick up the campaign so he could resume playing his character. The character was his favorite in twenty years of rpging and the campaign was also his favorite.(emphasis mine)
Why not? Why are you bent on stopping a player from playing a concept they would enjoy? Why are you only "willing to run" what you want? The game is supposed to be about having fun for everyone, not just the DM.
The above does not mean that I will not try to accommodate players to point within whatever confines I have set . For example, at the start of one 3.0 campaign, a player wanted to play a drow, but my setting had no drow (which was known up front). I did, however, have an island of elves whose religion was misunderstood by outsiders (and they did happen to have a spider spirit in their religion). I agreed to change their appearance to accommodate the player provided they still used high elf with the few cultural tweaks for the racial abilities. Spellcasting could be obtained via feat, taking an arcane class, or taking shaman (their religion was shamanistic based). The player was more than happy, because appealed to her about drow was their appearance and being outsiders.