Some go with the "gay" story - this was put round by Lou and the bodybuilders - men who cover themselves in oil and stand on a stage in a little pair of pants flexing their muscles while an audience of men scream at them tend to be homophobic for some reason
Some say it was because Batman was Bruce Wayne
Some producer didn't like the alliterative names Marvel always used, Peter Parker, Reed Richards, Sue Storm, Matt Murdock, Jonah Jameson, etc
Bruce Banner played by Bill Bixby was a bit much to take
Other they is that this is a different Universe so David Banner, the Medical Doctor who becomes the Hulk through the strength experiment, is not the same as Bruce Banner the Physicist military scientist who becomes the Hulk from the bomb accident
Kenneth Johnson needed a new name and David was the name of his son