Because guys are assholes and at the end of the day, we're all a little self-centered.
I'll admit that I've been the one to disappear, and I've been the one that someone has disappeared from. Granted, once I "grew up" a few years ago, I stopped trying to be a disappear-er.
I think a big part of it is that no one wants to be a bad guy. People don't want to say that they aren't interested. We all know that at the end of the day, there's nothing wrong with not clicking with someone, either... and that's the funny part.
Some people also put more stock into the exchange than others. I met this guy at a bar one night, and he was on vacation here in Chicago. So we chatted, I was hospitable. He had no intention to move here, visit again, come to school here, transfer for work, nothing. He wanted to exchange numbers, and I was like, "Sure, if you ever come back to Chicago, be sure to hit me up." Granted, I never thought it would happen, but I would've gladly met him out for a beer or two. Three days later, he's back in his home state and he's texting, and we chatted a bit, nothing substantial, no heavy conversation.... small talk. I didn't respond to a text because I was at work, and all I got back "Wow, I thought we really connected when I was in town, I guess you're just a typical asshole." No, young man, I was at work, I didn't respond after two hours because I was at work, and you're 1500 miles away. I'm not out looking for penpals. Again, we just looked at it from completely different angles.
But I'm interested in what you said, azndude89, what is a better way to rephrase "hey, sorry but I don't think I'm interested anymore" without leaving the door open to that changing? I think thats what also makes it hard. If you're not interested, you're not interested. If you say, "...I don't think I'm interested anymore especially with everything I have going on right now because I'm so busy..." opens the door too "Well, when won't you be busy, I'll track you down then?" Unfortunately, the only way to do it is be blunt, and people don't respond to that very well most of the time.