Here's an educated guess based on my experience with other publishers. They ran an analysis to see which of their monographs are most profitable (or receive the most citations,* or whatever other metric they care about). It turns out that there is a correlation between length and profit. Therefore they ask for a certain number of words.
It's almost surely not a hard requirement, they'll probably accept an 80k-word monograph if you adamantly refuse to lengthen the manuscript.
*That longer articles get more citations is fairly well-established by the way: see e.g. 1 2 3