It appears these results indicate the average age of the respondents more than anything. In general, people over a certain age tend to prefer speaker systems, those under a certain age tend to prefer headphones/personal audio.
I do agree there is a correlation with age, but I don't think that relationship is causal, and there are some underlying factors at play.
I do think younger people listen to music with headphones majority of the time, by choice, for a number interrelated reasons.
First one is tonal balance and bass response. You can get great sound out of a good pair of headphones with a decent DAC/AMP and EQ.
In order to get the same tonal balanace and bass response with a speaker system, you'd need a DAC, a decent amp, a MiniDSP maybe, a good pair of floor standing speakers, a few subs; you gotta get a decent measurement mic and fix your "room modes"; you need to pay attention to your ceiling, your carpet, your curtains and walls. Even if you don't do all of those things, you still need to put in a lot more money, time, effort and physical space to speakers than you would to headphones - a physical space you own most likely if you are gonna put all that effort.
And yet still, you need to be there, in
that room to enjoy it - you cannot take it to the office with you, or to a walk; you cannot use it on the plane, or by the pool while you are on vacation - which is the second advantage of headphones : mobility. You can get good sounding ones that work wireless even! (imagine that high-end speaker cable sales guy)
Third one, which is related to the first two, is time. I, for one, (not that I am
young) spend a lot more time during the day listening to music than I can afford to spend sitting in a somewhat treated room with multiple subs and floor standing speakers and carpets and curtains and whatnot.
In my view, in order to be able to do most of your listening with speakers, either you gotta be doing relatively shorter listening sessions, or you gotta have the money, the space and the time to sit in a room where your main setup is, which is probably your home, and must be able to afford spending a lot of time in that room. With all those in mind, I think it is not very surprising that younger people use headphones more.