Not that I can blame anyone. With these types of big bads you can't just make a start and hope that more people will join in as the fight progresses, and nobody wants to be the first person to run in and get annihilated. (Only someone well-geared and somewhat tanky really stands a chance anyway.) The encounter at Camp Vengeance also features the additional hurdle of having a step where you need to carry a certain amount of supplies from A to B but you can only carry one at a time, necessitating a certain minimum number of players to make sufficient progress before the timer runs out. Though I don't know if the encounter fails entirely if you fail that one step.
My pet tank and I did get Camp Vengeance done once by grabbing a guildie who plays dps and simply making a start on the not-too-deadly first step. Once we reached the supply carrying phase and others could see that there were multiple people working on it and actually getting somewhere, they slowly started joining in as well, and by the end we had sufficient numbers to complete the encounter successfully. Nothing attracts additional players like success, but it's hard to get things off the ground.
Selfie with giant dino in the back.
With King of Spines we got lucky as we noticed a player who was in the same instance shouting about wanting more players to take on the heroic. We figured that since we were already in the same instance, we might as well join in, and the group eventually grew large enough to successfully start and also complete the fight.
I'd love to know Crytic's thoughts behind this change though.