Because hardness is an important factor and hardness.
The softest hardwood.
A hardwood named quipo cavanillesia platanifolia is commonly reported as the softest known wood with an alleged janka hardness of 22 lb f 98 n.
Wood is categorized as either softwood or hardwood based on physical structure and makeup.
However the wood is omitted from this list for two main reasons.
The hardest commercially available hardwood is hickory and it is five times harder than aspen one of the soft hardwoods.
For example balsa wood is known as one of the softest and least dense types of wood yet it is categorized as hardwood.
Similarly wood from the yew tree which is one of the toughest woods and is harder than most types of oak is classified as softwood.
First it is virtually unobtainable and not commercially available outside of its natural range in central south america.
And while this example lists just some of the most popular hardwood species there are hundreds of varieties representing the north american hardwood population.
Although classified as a hardwood alder is one of the softest of all hardwoods and is more commonly referred to as a semi hardwood.
On a hardness scale alder ranks just above pine and poplar.