According to the Reading Teachers Book of Lists, of the 100 most popular (used most often) spoken words in American English, the question word "what"(there are six question words that are commonly used in journalistic and other research: what, who, where, when, why and how) comes in thirty-seventh. Only one other question word, "who," makes the list, and it is eighty-seventh (so who is certainly not on first). Why is what so popular?
Where, when, why and how deal with the places, times, reasons and methods "whos" do things and "whats" happen. This means that generally there will be a "what" or a "who" associated with each, but likely not the inverse. Of course that only explains why "who" and "what" get on the list in the first place. The reason "what" is ahead would seem to me to be directly related to the disproportionatly smaller number of "whos" (people) there are to "whats" (basically everything else under the sun).