Because certain published authors didn't get there, either.
Patrick Rothfuss, the author of The Name of the Wind and The Wise Man's Fear, posted on his blog about a week into November that he was doing NaNoWriMo. It was a long post about how he had thumbed his nose at NaNoWriMo in previous years, and it seemed like his reasons for finally deciding to participate boiled down to, "The fuck I can't."
Recently he posted again, about how he had failed to reach 50k, which was quite funny because of how "riled up" (his own words) he was in the previous post. He also posted about all the things he got out of participating. As he put it, he wrote 35k that he might not have (or at least not all of it) if he hadn't participated. He also said that he learned a lot, such as how much he can write in one hour, and that he doesn't need to block off three or four hours of time in order to get any writing done.
Great lessons, for all of us.
But really, don't feel bad if you didn't get to 50k. The real success is that you wrote anything at all, which is likely more than you would have written otherwise. And if it still really grates on you that you didn't make it to 50k, well, there is always next year!