At least part of the reason is that the interaction between a lasgun and a shield would cause a powerful explosion that would kill the shooter too. No one wants that and no one will give up their shield, so they had to go back to melee weapons.
> However, a great reaction against computers has resulted in a ban on any "thinking machine", with the creation or possession of such punishable by immediate death.
tl;dr - Machine intelligences existed in Dune history, were discovered to be secretly controlling humanity (through abortion under false pretenses, forced sterilization, emotional/social control, and other ways), then were purged and replaced with a religious commandment: "Thou shalt not make a machine in the likeness of a human mind"
The reason nobody tries to use the lasgun-shield interaction as a weapon is because the resulting explosion is indistinguishable from a nuclear weapon, and the Great Convention prohibits the use of nukes on human targets.
Just the perception of having used a nuclear device would result in the House which did so becoming public enemy #1 and being eradicated by the Landsraad and Sardaukar combined.
@Potro: If you liked the movie, read the books. I don't read a lot anymore, but during sick leave I started with the first book. Didn't stop until I finished the main story, including the sequels by Frank Herbert's son about a month later. That's like... uh... nine books?