Sampling on campus. Why this sampling method is almost certainly biased.

Actually there's several reasons. The main idea is that it's not a truly random sample. This researcher is limiting herself to students who visit the student center and do so at that particular time of day. What about students who don't visit the student center? Or those who do so at alternate times of day? Add to that the fact that by selectively questioning certain individuals she's handpicking her subjects. Recall the definition of "random" ; picking your sample in such a way that every member of the population in question has an equal probability of being selected.