Paladins are pretty strong as far as small scale fights go, but they fall off pretty quickly when fights get into the 5v5 or 10v10 range. Our defenses are purely linear, my healing vs your damage. If I outheal your damage, I win, and if you out damage my healing, you win. We have some cooldowns to help slow their damage or boost our healing for a short while, but they're still tug of war, which will inherently fall off in larger fights.
By contrast, the survivability of other classes is tied to things like range, mobility, visibility, crowd control and so on. Things like that can be combined with each other to escape from danger and return to safety to regain the upper hand, something Paladins lack.
For example, each horrify for Warlocks is a free form of CC that brings them that much closer to resetting the diminishing returns on fear. Then their fear puts distance between the warlock and their target, which can only occasionally be closed again quickly. Mages have iceblock to let them wait for blink, frost nova and barrier to come back, same with hunters and deterrence providing a delay for another scatter/trap, disengage, explosive trap or whatever else they need.
These classes have emergency survival skills that will cover a moment of vulnerability and allow them to escape. Paladins are one of the few that have no such method of escape. They rely upon their healing and damage mitigation, but they can't shut the flow of incoming damage off save for once per 5 minutes. The difference is that there are no cooldowns or escape tools the Paladin is stalling for.