Because it doesn't provide solid flow all around the tank. Using lily pipes? Tiny amount of flow. Using a custom spray bar can help but there are still a ton of dead spots and circulation pumps are necessary.
My general rule of thumb: 2211 for 5gal or less, 2213 on nothing larger than 10gal, 2215 on 12-15, 2217 on 20-25.
I find the 2217 is perfect for a 20long. Seems to be consistent with what a lot of Planted Tank folks use.
I use an Eheim 2217 on a 40 gallon breeder, it has good flow. I don't know why anyone would think it doesn't.