Because of what happened to all the cool computers (read Dan Abnett's Dark Mechanicum for the 30K version of this tale) 40K can't suffer even a Commodore 64 to live.
Machines were twisted by the powers of Chaos, and made evil. It is unclear whether this has happened twice to Humanity or not. An initial golden age of expansion flung Humanity to the furthest reaches of the galaxy, only for a dark age to settle in atop that, only to be ended by the unifying conquests of the Emperor. During the Horus Heresy, the digital Chaos curse infected the Mechanicum on Mars, slaying many and turning some into grim servants of Chaos, due to their extensive bio-cybernetic enchantments. The curse itself in 30K had been imprisoned in the tomb of some ancient and accursed techno-witch, who had been buried deep within the surface of Mars long ago, presumably before or with the arrival of the Emperor, who knew where the tomb was and kept secret its location from all others. The evil of machines from this point on necessitated the creation of sanctified servitors, the technology for which already existed throughout the Empire as punishment for certain classes of criminal offenses.
Bottom line, all the parts of the brain that were the person are dead. Some things remain: pattern recognition, short-term memory capacity, but only the barest rudiments, to make it an effective lobotomized slave, and to keep it safe from the corruption of Chaos.
P.S. This is based on Games Workshop lore for Warhammer 40K and 30K. At the end of the day, the DM is bound by nothing. But if they want to keep their campaign canon, they'll need to read the canon.