The humans outlawed, religiously prohibited, and feared, non-human computation.
The Butlerian Jihad was an epic war against AI beings that had enslaved humanity. After the human victory, the humans outlawed non-human computation. Hence various guilds of human computers (e.g. the mentats) formed in order to cultivate the human ability to perform the sort of computations that computers had performed for them before the Jihad.
The Butlerian Jihad is an event in the back-story of Frank Herbert's fictional Dune universe. Occurring over 10,000 years before the events chronicled in his 1965 novel Dune, this jihad leads to the outlawing of certain technologies, primarily "thinking machines," a collective term for computers and artificial intelligence of any kind. This prohibition is a key influence on the nature of Herbert's fictional setting.
From the glossary in Dune
JIHAD, BUTLERIAN: (see also Great Revolt) — the crusade against computers, thinking machines, and conscious robots begun in 201 B.G. and concluded in 108 B.G. Its chief commandment remains in the O.C. Bible as "Thou shalt not make a machine in the likeness of a human mind."
From Dune Messiah
"From the days of the Butlerian Jihad when 'thinking machines' had been wiped from most of the universe, computers had inspired distrust."
In God Emperor of Dune, Siona Atreides presences Leto's "Golden Path"
He knew this experience, but could not change the smallest part of it. No ancestral presences would remain in her consciousness, but she would carry with her forever afterward the clear sights and sounds and smells. The seeking machines would be there, the smell of blood and entrails, the cowering humans in their burrows aware only that they could not escape . . . while all the time the mechanical movement approached, nearer and nearer and nearer ...louder...louder! Everywhere she searched, it would be the same. No escape anywhere.
If you accept Brian Herbert's prequels as cannon, then a small groups of humans used a network of artificially intelligent computers named Ominus in order to enslave humanity. A century later, Ominus enslaved the humans who used it to do so, and so enslaved all of humanity. That slavery lasted nine centuries. It ended when Manion Butler incited the slaves to revolt; thus beginning the century long, but ultimately successful, Butlerian Jihad.