Because it's not just about first place votes.
Imagine a poll where you award 25 points for 1st, 24 for 2nd, 23 for 3rd, etc. and that for simplicity there are only three voters. If one voter puts ND #1 and the other two put them at #2, ND gets 73 points (25+24+24). If the other two voters put us at #1 but the third puts us at #4, then we have 72 points (25+25+22) and end up behind ND in the poll even though we have more first-place votes.