<p>I was asked this question during an interview. They're both O(nlogn) and yet most people use Quicksort instead of Mergesort. Why is that?</p>

Actually, QuickSort is O(n
2
).  Its
average case
running time is O(nlog(n)), but its
worst-case
is O(n
2
), which occurs when you run it on a list that contains few unique items.  Randomization takes O(n).  Of course, this doesn't change its worst case, it just prevents a malicious user from making your sort take a long time.
QuickSort is more popular because it:
Is in-place (MergeSort requires extra memory linear to number of elements to be sorted).
Has a small hidden constant.