Is there a reason why WHM is the weakest healer in all fields?

[CrimsonGunner]

Because it would be even less fun to buff the healing even more.

WHM may be the worst healer numbers wise and people say it's plain and boring - but without an overload of off-GCD heals and solutions to every problem it is probably the most fun healer to actually play.
You even can run out of mana if you play messy.

it isn't due to stuff they actually thought out as part of its gameplay but a result of neglect and disjoin patching from the devs.
they haven't really thought about anything that relate to whm gameplay really.

your right that part of the fun is playing with a kit that have its strength and weakness but those aren't a result of proper gameplay they thought. that is how it seems to me.