I think shyness is the main reason. There are some people that just cannot hold eye contact without getting creeped out. There are other reasons as well. For example, a guy who I had a crush on always made eye contact. I would look at him but then look away and look back again. There were one or two times where he caught my eye and held it for an extended amount of time. I liked it at first but the longer he held the stare, the more I was thinking that he was after one thing only. It kind of was like he was trying to send me a signal that he was imagining us together in that way while he was staring. I surely got the message. It was kind of scary because I did like him but wanted it to be more about getting to know each other and not just about sleeping together.
We can be shy at first, but honestly, sometimes we don't want it to seem awkward, or we jst play hard-to-get, but most the time, we don't hold eye contact because we don't want to look at you, we want you to come to us, its kinda like just catching your eye for a second, kinda intriuges you to come say hi? Try that,C: