Because you don’t have very much earned income. The original theory behind the credit is that it was a offset of your Social Security tax, because everybody, no matter how poor, pays 6% Social Security tax, even if they are in the 0% income tax bracket. The earned income credit is calculated on a sliding scale; if you earn very little, you get very little credit, because you didn’t earn much, and if you earn a lot, you get a very little credit because you don’t need it anymore. The maximum credit is in between.
