Don't worry, I will give you context.
Paladins are an Alliance exclusive class, they fight using the powers of the Holy Light.
Why are there no Paladins in the Horde?
No race of the Horde cannot summon the Light.The Orcs: They are mainly shamans, in addition to the murky past with the Legion and the Corruption that still remains in their bodies.
Undead: Although they may have been Paladins in the past, that power was lost upon dying and being raised as Undead, as the evil they possess in their bodies is not yet gone, and the light is deadly. For any Undead creature, without exception, summoning it would be torture.
Tauren: Although the Tauren are druids, they do not worship light, any type of light, they are more shamans and druids, faithfully devoted to nature and its blessings, there is no place for sacred light in their culture.
Trolls: Trolls have never worshiped the light, they worship the loa, who are like gods to them, their culture is based more on war and the elements, and to a certain extent on witchcraft and spiritualism.
Goblins: It is practically impossible for Goblins to summon the light, they are not devoted to it and have never had a culture like that, goblin culture is based on greed, merchandise...and the trafficking of things. They are merchants and inventors by nature, seeking to exploit the merchant potential even of stones.
Why they are a exclusive class of the Alliance:
The human race was the first to begin worshiping the Holy Light, they were the ones who started the custom until they formed the Church of the Holy Light.
This custom was gradually passed to the High Elves with the founding of the Church of the Holy Light in Lordaeron, as although they worship the sun, they do not see it as a god.
This belief was also passed to the dwarves from the first interaction between Humans and the dwarf race, being adopted and creating several dwarf paladins.
All the Paladins are part of the "Order of the Silver Hand", it's an order of Paladins founded in Lordaeron by Alonsus Faol, every Human, Dwarf or High Elf is part of this sacred order.
But you will think, the Order of the Silver Hand was not destroyed in the Third War?
Yes and no
Some remaining members founded the Scarlet Crusade, an organization who wanted to restore Lordaeron, but after noticing the corruption inside them, some of them abandoned the Scarlet Crusade and founded the Argent Dawn, other members traveled to Stormwind after the destruction of Lordaeron refounded the organization in the Kingdom of Stormwind, their main base is in the Cathedral of the Light, what is shared with the Priests, and you are part of that new generation of Paladins, you're part of the restored paladin order and follow the Legacy of Uther the Lightbringer, the last Supreme Leader of Order of the Silver Hand, and most important, you fight...
for the Alliance!
Uther the Lightbringer, Alexandros Mograine, Saidan Dathrohan and Turalyon, members of the Knights of the Silver Hand.