@TomeBinderI suppose it couldn't hurt to have it pinned to the top of the arena thread. I think most of the reason it's not, is because it's not exactly something that is enforced upon people. They can fight however they want, but most people use at least a slightly modified version of both the 10-tiers and T1-type of a fighting format out of familiarity with it. Some people go free-form, which is cool too, but requires a much more carefree attitude. Some people are in it for the challenge, and thrill of victory (or impressive defeat) - And for those, it is often better to have a good power level reference.
A lot of people who use the arena are familiar with how things are done around here. Which might be one of the arenas biggest problems. The people who use it are people who have been around for a while, or at least have a basic understanding of the most used format before entering, so I suppose it ended up as a silent "We all know what we're doing, and how we're gonna do it", so nobody put anything up (officially) for those who didn't have a clue. Would be my guess, but that's a really big shot in the dark. Not the easiest type of place to get into, but the people are nice. I've been around for about 4 years, and it's kind of always been kind of like that. Same thing over at roleplay gateway, where I started out, but they've added a few stickies over the years to help out newbies.
The tier list could at least be a nice thing to have as a sticky around here. Most people at least know of it, and it's good for reference, although some people prefer to use their own modified version of the tiers. One of the first persistent arena threads that was made on this site after it was re-created (it crashed pretty severely a year back) - used a 3-tier system, that was technically nine tiers. Low tier, mid tier, and high tier. Then there was Low-low tier, Low-mid tier, Low-high tier, Mid-low tier, Mid-mid tier, Mid-high tier, High-low tier, High-mid tier, and High-high tier (worded better, but I can't recall the details) - and basically, everyone within the main tier (low, mid, high) would count as eligible opponents for each-other. Didn't work out well. Partly because of the system, mostly because of the people.