This is Pokemasters response the last time this was asked:
This has been answered before. The Pokedex pages and the rest of the main site is for factual content. Whereas tiers are someone's opinion. Adding them wouldn't be a big deal but Smogon's tiers change regularly so keeping up to date is a pain.
You can just go to Smogon in one tab and look at the tiers, e.g.
http://www.smogon.com/bw/tiers/ou then checkout each Pokemon on our
Pokedex in another tab.
Or check this page: http://www.smogon.com/bw/pokemon/ which lists
every Pokemon with their tier. Nothing difficult about that whatsoever
Probably won't add this.
1.Tiers are nothing official so anyone can disagree if they want.
2.The ones that come closest to official in the competitive community are on smogon.com, so if anyone wants to know they can just go there
to see them.