As of Morrowind, there have been ten playable races. But there's only Nine Divines, and Nine is one of the most important numbers in magic. 10 - 9 = 1, so where's the tenth divine? It can't be Trinimalacath, because I thought he was a new name for Zenitharkay. It can't be Tsun either, since he's in Sovngarde. And we are not talking about Sovngarde right now.

Then again, there's also the Nine and Nineteen Voids that Sir Haskill of New Sheoth once spoke of, but that means some races are worth more than others. And I'm pretty sure Dunmer are worth at minimum a 6, with the lost house of Voryn Dagoth factored. And according to the Lunar Lattice, the Khajiit = 16 minimum(!). So we're getting over the limit here. Perhaps you guys can figure it out...? Or perhaps blessed Sheogorath can help; I can get cabbages at Carlotta's stall, but I'm short on yarn.

EDIT: After checking on http://www.uesp.net/wiki/Lore:Races, I realized any way you slice the buttered toast, there's just too many races. That would've led to some unstable racial nuclei. So my only assumption is that many are undergoing mythic decay.

The Eight and One Divines, accompanied by the Outcast who was.

It used to be eight in the pre-Morrowind days, before Cyrodill got its own race and Orcs were recognized as people.

There might be ten playable races, but remember that there are far more then 10 races:

Imga, Sload, Giant, Goblin, Falmer to name a few. There might even be more races we don't even know about, taking into account Akavir and other landmasses we haven't heard of.

there are more then ten races on tamriel , besides do we really need some mythical reason for ten playable races, not everything needs to have meta mythical reason for existence