Why are girls so ruthless in online dating?

I have a profile on plentyoffish.com I'm a young 25 year old african american male. But everytime I send a first message to a girl that I'm interested in, I never receive a response back. I have a profile picture of myself smiling and being positive. Because that's what attracts women right? A positive attitude? Girls always say that they're tired of the messages they receive from jerks talking about getting they're number and hooking up for . Basically from "jerks". But I always send them positive messages commenting on their interests and even complimenting them on they're smiles. But throughout all of this "positivity" no response. Not even at least a thanks but no thanks type of message. I think race plays a role in it. I put up a picture of a average white guy my age with my same words and I get responses back left and right from those same females who wouldn't even bother sending a response to any of my first messages! It's crazy. It seems like girls are raised to date every race except black men. I feel like I'm not ugly and have had girls tell me that I'm attractive in real life. But, I feel like I'm cursed being black.