It's a measure of exceptionalism, somebody possessing biological qualities and/or physical characteristics few others do. For example, I like really tall and really short ladies, incredibly busty and completely flat ladies - because each of these physical characteristics makes the girl in question more unique, rarer, and therefore special and desirable. Essentially, it's about how 'lucky' I am as a partner to have them be mine.
Something that drives up the 'uniqueness factor' is when two things that rarely combine meet against all odds, in this case; youth and fertility. (Oppai lolis are great for this reason too.) If this is then further made a point by having the girl exhibit maternal qualities despite their age, this once again drives up the uniqueness.
There is also within me the caveman desire to continuously give in to my urge to procreate, and if the willing partner is more youthful this would in turn give us more time to have more babies.
Lastly, lolis are cute.
Tl;dr it's about the juxtaposition of youth vs fertility