The importance of head protection in every career or recreational pursuit is well known.
It took a long time to make them mandatory (in some parts of the world) on bikes, it is the reason cars have tons of airbags (the side airbag was invented after they worked out people were dieing of head injuries from side impacts).
In those days it was uncool to wear helmets on bikes, in some cases it still is considered uncool, but if you are prepared to humiliate yourself on a Harley with all that leather and tassels then a helmet is the least of your worries.
In recreational flying and training, helmets have the same problem, no one wants to be seen in one, hot environments and no air-conditioning don't help.
There is also the the thinking that accidents are few and far between and a good helmet costs a few flight hours so why waste the flight time.
The culture isn't going to change until regs require it, and I doubt regulators will as there just aren't enough people affected to justify it.
Perhaps insurance companies may start insisting on one being worn.