okay so i read about this a long time ago and just remembered.

one time in an interview between fans and J.K. Rowling, she told the audience that hogwarts had about 1000 students attending. that means that there would be only around 35 students in each house each year, but there only appear to be 10 students in almost every house in Harry’s year

i believe this is because around the time most of these children would have been conceived, was the time Voldemort was at his peak in dark power. many adults who joined together to help fight against him, decided it wouldn’t be wise to bring a child into this mess, causing a “baby drought” if you will.