It could be that, or it could be just that they really couldn't make it. However, I'd be very wary of someone who stands you up frequently, as they are unreliable
My first read thought you wanted to know why men stand up for a date! My sister's father in law did that. Everytime a woman entered the room, he stood up and offered her a chair. If she didn't want to sit, he stood along with her....Man, so old fashioned, but so reverential. He was a Gentleman to his very bones!<br />
To answer your question...They stand you up because they are bad mannered and good riddance to them.
It could be that nerves got the better of them, and they got cold feet. But its common courtesy to let someone know that they will not be able to make it. If they haven't got the decency to do that, then they were never going to be worth your time. Its little things like this, that make you realise what the person may really be like. If they can't contact you about something as simple as meeting up, how reliable are they likely to be in general? Its not a good start for something solid. Sounds to me like they did you a favour. ;-)