I'll tell you why. The dungeon finder tool uses average ilvl to decide if you are geared enough for a dungeon, but it doesn't take into account whether the gear is green, blue, or purple! I've reached the point, now, where I can't queue for randoms.
I've learned a couple of things, though, about how ilvl is computed. If you go to the dungeon finder and select "specific Cataclysm dungeons", you can mouse over the individual dungeons and see what the game considers to be your current ilvl. Here are two interesting things about that computation:
- It includes everything in your bags. When I get an item, it goes up. When I disenchant an item, it goes back down.
- It doesn't correct for duplicates. I had two waist items, and then I disenchanted one of them, and my level went back down.
The good news is that the ilvl requirements are rather mild. However, I wish I'd hung onto more quest greens now. I'll definitely do so on my later characters.