If I remember correctly what's written in the King Books, at the beginning D'ni wrote hinabitated Ages. But with the passing of time they closed themselves more and more (the 2 D'ni Wars were not a help and also various problems alogn the millennia) and I think that during one of the Kings reign (Kerath?) they decided that writing hinabited Ages was illegal.
Found it. But it doesn't explicitly say they were made illegal
Kerath Book wrote:
By the end of his reign, Kerath had convinced a majority of D'ni of his own beliefs. Most claimed to be followers of Gish and his writings, and most viewed the outsiders as a threat. "If not now, then soon," Kerath often said.3
After his death, Kerath's words would be proven true with the onset of the Mee-Dis War. Outside factions would invade with attempts to destroy the Ink-Making and Book-Making Guilds and almost succeed. By the time the war would end, there would be few left who did not follow Gish and thus believe in the end of most outsider involvement. The discovery that conservative factions had led to the start of the Mee-Dis War would come much later, when it was far too late to alter the conservative trends.