Consider that the locket had a lot of physical protection, the secret entrance, the lake full of monsters and the requirement to drink poison to get it. As far as we know one person died in the course of stealing it. So while you can always argue that Voldemort could have added more layers of protection even he has to stop somewhere.
Equally Harry et al knew that the locket had been in circulation for a while since it was stolen from the cave, they eventually got it from Umbridge who had been wearing it for a while so it was pretty safe to assume that it didn't have a similar lethal cruse.
On the other hand the ring was hidden under the floorboards of derelict house so there was some remote chance of it being discovered by accident in which case a deadly curse makes sense as a final level of protection.
Voldemort may well also have reasoned that by diversifying the protection on each horcrux he reduced the chance that any one person would have the ability to find them all.