Astoria can't have any more children.
By the time the Mediwitch realized it was going to be an unusually difficult delivery it was too late to transfer Astoria to St. Mungo's Hospital for specialized help; either Floo or Apparition would severely damage her and the baby, and a witch in labour could not be transported on a broom.
The Mediwitch did the best she could but Scorpius almost died in the process. In a similar situation Muggles do something called an emergency c-section, but in the wizarding world the only way to save both mother and baby was to cast a spell that left Astoria unconscious and unable to conceive again.
When she woke she took the news hard; she had wanted more children and couldn't believe something like this had happened to her. She wept freely even as she held the precious bundle in her arms with extreme care, being comforted by Draco who felt grateful that both his wife and his son were alive and heartbroken that their family wouldn't grow any more. Even the Mediwitch unprofessionally broke down in tears, feeling responsible for this outcome.
Draco and Astoria are very overprotective of their son. They can't help it; he's the only one they'll ever have.
A/N: My explanation for the single child, though canon Draco and Astoria probably chose to have only Scorpius.