Different post-production practices. The original run was edited on film, so masters can be made from original 35mm camera negatives. By 1988 some production companies (notably Paramount) were starting to edit on standard definition NTSC video, which obviously limits the possible quality.
Stark Trek: TNG fans are lucky in that new HD remasters have been done, but it's (a) very very expensive and (b) impossible if the film elements no longer exist. :(