By the time of the Cursed Child, time turners
are almost non-existent.
but we discover that
Hermione still has one, as does Malfoy.
It's then made pretty clear that once the time turners are
smashed during the Battle of the Department of Mysteries, there is no chance of getting another (not counting Hermione and Malfoy's).
Why couldn't someone just make more? I mean, the already existing ones had to have been created somehow! What might be the case?
There are strict laws surrounding the creation and study of time-turners and time magic in general. This presumably includes not making new ones after their existing stock was destroyed.
‘Finally, there were alarming signs, during the days following Madam Mintumble’s recovery, that time itself had been disturbed by such a serious breach of its laws. Tuesday following her reappearance lasted two and a half full days, whereas Thursday shot by in the space of four hours. The Ministry of Magic had a great deal of trouble in covering this up and since that time, the most stringent laws and penalties have been placed around those studying time travel.’
Out of universe, their destruction was a way for JKR to write herself out of a hole. Having someone start making new ones would be like picking at a scab.
I went far too light-heartedly into the subject of time travel in Harry Potter and the Prisoner of Azkaban. While I do not regret it (Prisoner of Azkaban is one of my favourite books in the series), it opened up a vast number of problems for me, because after all, if wizards could go back and undo problems, where were my future plots?
I solved the problem to my own satisfaction in stages. Firstly, I had Dumbledore and Hermione emphasise how dangerous it would be to be seen in the past, to remind the reader that there might be unforeseen and dangerous consequences as well as solutions in time travel. Secondly, I had Hermione give back the only Time-Turner ever to enter Hogwarts. Thirdly, I smashed all remaining Time-Turners during the battle in the Department of Mysteries, removing the possibility of reliving even short periods in the future.
