It's nothing to do with the hardness or softness of the wood although usually hardwood is much more resilient than softwood. Balsa wood is a hardwood and we all know how soft that is.
Softwood grows fast and has a more porous cell structure which left untreated will rot quickly. Some 30 years ago pressure treatments were invented that mean softwoods are impervious to damp and fungal decay.
This treatment called tanalising can make softwoods last for 40 of 50 years in damp ground conditions, which means many softwoods can last as long as some hardwoods
Appearance is the main difference between hard and soft woods. They do smell different. Hardwood decks are more expensive this usually tends to focus people's idea's of which deck to have.
A softwood deck costing £2,500 would cost £3,500 in Iroko £6,000 in Oak and £10,000 in Teak. They all give you the same space obviously they feel different but for the extra money they need to.
There are two types of wood in the world. Softwood comes from evergreen trees, mainly pine. Hardwood comes from deciduous trees. Some hardwood is soft (eg. balsa) and some softwood is hard (eg. pitch pine). Hardwood is not better than softwood, it is simply different.
Hardwood decks look different to pine (softwood) decks - and they cost different. Hardwoods come in many different colours and they always cost more than pine. Sometimes 50% more. Sometimes 300% more!