I know you're going to be thinking, "why not just go to smogon?" But I don't ever use smogon. I've only been on it twice, and I thought that this would be helpful for people who don't even know what smogon is. It would be convenient and it would stop questions like, " What tier is ____ in?" We would use smogon tiers because you use a showdown server. It's just a thought, but I'd be quite happy if you did it.
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 :)