Daughters are more likely to give you grandkids than sons are. If you had a choice to pick between a 3/10 daughter and 3/10 son, the former would have a higher chance of reproducing than the latter just by virtue of foids being able to select their partners. Considering how most people are of average attractiveness and the minimum threshold needed for a man to be acceptable as a LTR by women keeps increasing, the incentive for having daughters grows with every passing day. Because it becomes harder for average or ugly men overtime while conversely it gets easier for women of all levels of looks. Having sons is less likely to continue your genetic line than having daughters nowadays, especially if you're of average looks like most people in the population are just due to statistical law.