Obviously some folks on Earth must have had some problems with their David units, which more than likely caused the Weyland Corporation to go back to the drawing board and produce the new Walter line of robots.
To make a comparison to previous Alien history, David is like Bishop in Alien 3, while Walter is more like Bishop in Aliens.
Who’d have thought the robot we were all sold four years ago would start to creep people out, causing them to distrust their David units.
This is certainly a huge step from the David model we saw in Prometheus, as he was surprisingly able to carry out human traits such as emotional displays, dyeing his hair out of vanity and, most distressingly, lying about his true purpose on the titular ship’s mission to LV 223.