1) General Vulnerability to Shadow Magic-
Almost half of the armies in the game have access to Lore of Shadow, including most of the stronger ones. It is taken by nearly every army with access to it. Three of the spells in the lore are essentially game enders for Ogres, and the rest are not particularly pleasant. Plus it can do its work from outside effective hellheart range. Any army with serious weaknesses to the most common and powerful lore of magic is not really in the top tier. Lizards get a pass on this because the combination of Becalming and two scrolls lets them defend it better than anyone. Ogres are basically hoping for a 5 roll on the heart (or a dumb opponent) for a one turn reprieve.
2) Leadership issues-
One thing the best armies have in common is strong leadership. Skaven depend a little more on their general, but they can reach LD10 with a wizard without resorting to a magic banner all over their army. Lizardmen have cold blooded. Dark Elves have LD so good that they can afford to send their general away from their entire army. The need to cluster our forces around a single point greatly limits us, as well.
3) Abundance of Underpriced units-
Even if you buy the idea that Ironblasters are undercosted (I don't), thats one unit in the entire book. Dark Elves, Lizards, and Skaven are chock full of massively undepriced items in every part of their army, especially in their core selections. The basic Slaan is cheaper than a naked L4 Slaughtermaster, with a built in discipline, 4++, and ton of special rules. Warp Lightning Cannons are vastly better than the Ironblaster and are very cheap. The Hydra is but one of a bunch of powerful units that are inexpensive in the Dark Elf book. Ogre's do not really have anything like that going on in their book.
4) Dirty Tricks-
We have the Hellheart and Deathfist (which is debatable for usefulness). Take a casual stroll through the tactics threads and you find a veritable cornicopia of combos and shenanigans that the top three armies have at their disposal, in all phases of the game. Our best tricks involve the magic phase, mostly in defense, which while good is not in the same ballpark as Sac Dagger, Skitterleap Orb Toss, or even the ubiquitous extra dice casting loremaster Slaan.
5) Matchup issues-
We do well against Lizardmen and alright against Skaven due to some peculiar matchup quirks. We hit the wall against Warriors of Chaos and struggle against some of the weaker armies out there. While Beastmen are a rare bird at tournaments, Dark Elves and Warriors of Chaos certainly not. Even if the chosenstar is not in the mix, basic Chaos Warriors of any stripe just pulverize Ogres and they are one of the most popular armies at tournaments. Skaven, Dark Elves, and Lizardmen primarily worry about running into each other.