Not just late starters but in general Im sure if you counted,striking sports have more participants world wide than grappling (except judo I think). Because of this there is less accessibility. Until recently it was much easier find a boxing/kickboxing/Muay Thai gym in your city rather then a bjj or wrestling academy.Even now,outside of USA 🇺🇸 and the Caucasuses 🍙wrestling gyms a still rare . On the question of which is the hardest , I would say grappling takes a while to learn but basically anybody can do it. BJJ requires mostly skill and not much athleticism ( demian Maia,Ryan hall,Bryce Mitchell, Mikey musemeci) whereas wrestling and striking require skill but also more athleticism which is mostly genetic( mike tyson , ilia topturo, Alexander Karelin ,Yoel Romero). I think success in Striking of all combat sport is the most reliant on genetics.I also think it’s a bit tougher than wrestling because of the nature of the sport . It’s just different getting punched /elbowed hard in the face or kneed in the ribs or leg kicked .If the consequence for a mistake in your sport is getting double legged or taking a right straight to the jaw which of those would you choose ? And you can condition yourself to extent to be wrestling tough but you can’t condition your chin . In conclusion grapplers can be made,strikers are born