Being MT isn't necessarily about being the best or putting in your dues. There has to be a slot for you to fill. If there is no opportunity for the spot, you won't get it.

I usually topped my guild's DPS heartily on my SPriest all through BC and up until ToC where scaling just clobbered me. I also started tanking in BC on my Druid mostly because my guild went through a rough patch and a split, so raiding wasn't possible for a while. Threat being what it was for SPriests in BC I kept a really sharp eye on the tanks I ran with and had a pretty good idea what they did when. My runs went well, although I put my Druid to the side when Wrath came out.

After I got clobbered by scaling I starting working on getting a DK together to tank, but
the problem was there already were about 4 other tanks, all better geared than I was. There was no reason for them to pass on gear upgrades when they already had more experience tanking, and there was no reason to use my tank since I was barely tipping the defense cap... so my my DK tank languished. I started leveling my Druid, but unending grind to collect piles of Arctic Fur and Nerubian Chitin for intro gear just exceeded my patience. I then started healing Disc as my SPriest's dual-spec, and it was honestly a lot easier than I thought it would be. I never healed raids, but I healed some rough 5-mans with my only previous experience being Wailing Caverns.

The point is, I knew the fights, had a good grasp of aggro mechanics, knew how to gear, had tanked for a while before, but there were just too many tanks that already were more established. There was really no way for me to step in. Either I could gimp a raid by MTing in my crappy gear, or gimp the raid by DPSing on offspec and hope tanky pieces that weren't upgrades dropped that absolutely no else wanted or could use. I did MT some Naxx runs, and I did find them easier than a lot of 5-mans at appropriate levels, but I could never overcome the gear edge and just sheer number of other tanks.