There are many reasons not to vote, even though you totally should. Here are the most common (via Brad Plumer and Kay Steiger):

Some interesting details from the Census Report that came up with the data:

1. 27 percent of Asians said they didn't vote because they were busy, considerably higher than the national average of 17.5 percent.
2. 15 percent of whites said they didn't vote because they didn't like the candidates, twice as high as any other group.
3. Young people (ages 18-24), Asians, and bachelor's-degree holders were most likely to report registration problems.