this may be what finally ends this... this is what I have been asking for, please walk us through step by step how having orcs in krynn ruined it...
That's going to be a bit difficult for me, in that a) I haven't looked at anything Dragonlance in about 30 years and b) I've no real idea what they're trying to do with it now.
what is the slope? how does the wedge work? This is what I have been asking for.
But this, in a different context, I can answer easily.
It's all about precedent and fairness. What's available to one player must be available to all.
Say for example that I-as-DM only want the core seven* species as PCs in my game, and make this clear up front to the players. Then a player comes to me with some interesting ideas for a Dragonborn he wants to run, and talks me into letting it happen. The thin end of the wedge is in, and now I'm on that slippery slope, because next time a player wants to bring in a new PC I have to allow Dragonborn as an option because I've already set that precedent with the other guy. Lather rinse repeat with various other species and suddenly I'm running a monsters campaign I never wanted to run.
Now I could (and do) gate such unusual species behind very difficult dice rolls in order to keep them rare and avoid the existence of one setting a precedent allowing a lot more, but whenever I mention that idea elsewhere in here I get flayed for it.
* - Human, Dwarf, Elf, Gnome, Hobbit, Part-Elf, Part-Orc.