Probably because they were using well-circulated currency for their lower prizes ($5, $10, etc...), and they were afraid of contamination as the prize appears to be touching the liquid itself.
A little extra cocaine wouldn't impact Coke though.
Weight. You need to approximate the weight of a full can using less liquid.
Since they specifically don't want people to drink it (contamination, perhaps), how would that get them closer to the objective?