If you search for 'CoD 4: MW' on this site, you can look at speedruns done on Recruit, Regular, or Veteran difficulty. My question is, why are we not given this option for other CoD's? For example, CoD World at War does not have the option to look at Speedruns done on Recruit, Regular, or Veteran. Why is this the case? Any way of implementing this feature into the site?

that has nothing to do with missing site features, the moderators just didn't set the leaderboards up the exact same for whatever reason

This is a question in the COD series forum or the forums for each individual game, not the site wide Introduction forum lol. If you want to add categories you need to ask the mods (and having a run ready would be good too).

You should get in contact with the mods themselves about this There is usually a reason why things like this are done. If not you could just suggest it and make it a thing