This is of course totally anecdotal, but a lot of people I know who've quit brewing said something along the lines of "it's just too much equipment and stuff to have around." Which to me, along with what a lot of other folks have pointed out, is a great argument for small batch brewing.
Back when I was in grad school I'd do 10 gallons nearly every week and it'd disappear because, well, grad school and lots of people who like free beer.
Nowadays I typically have one beer after work if that, so it'd take me months to go through even 5 gallons, which is a big reason that when I started brewing again I started with 1-gallon batches. That way I can experiment more, and more frequently, without having so much beer around that I could never drink it all.
I'll probably scale up slightly to do maybe 2-gallon batches of stuff I really enjoy and get some 2.5-gallon kegs in a kegerator, but unless I know I'm brewing for a party or to give a bunch away, I can't imagine doing 5-gallon batches again, let alone 10.
And even though I have a dedicated brewing space I really, really enjoy the smaller scale of all the equipment. Easier to store and clean, easier to manage, and the whole process just feels less overwhelming.
Just throwing all this out there as food for thought. Makes me wonder how 5 gallons became the de facto standard batch size to begin with, and how many people if they scaled back from that might find themselves still enjoying it.