WHM is stuck in a spot where it's bad but popular because the alternative, AST, despite being a powerhouse just isn't fun.

The devs only see that AST isn't popular and WHM is, so they buff AST and assume WHM is fine. AST continues to be bloated and not much fun to play but buffed to insane levels, WHM becomes even worse by comparison, and the cycle continues.

The devs seem incapable of realizing that the only problem with AST is gameplay and WHM is only hanging by a thread because they won't fix AST.

The other reason WHM is staying afloat is because healing is ridiculously easy, so even a terrible class can pull through without GCD heals. That's not good game design. From P3S WHM popularity suddenly plummets.