One spring is cheaper than a double plus a damper. Simple as that.
A BH spring has fewer resonant frequencies, but not "none". They avoid surge and bounce for less cost.
OEM engines are built to price point.
Spring life depends largely on the mass of the material involved. A smaller spring is stiffer, but can't take as much stress. Larger wire diameters can take more stress and increase stiffness. For heavy use applications, a BH would be going backwards. Multiple nested springs will always have more mass. Once a BH is made from thick enough wire, it won't fit..
BHs thus require lighter valves, and rely on their lighter retainers. Then the lobes have to be engineered along with the rockers to control the load and unload rate to prevent spikes which might exacerbate surge.
All that changes when one moves into high output builds. Bigger valves, faster ramps, higher lift - they all work against a BH design..
