Node running cost. In england it's expensive electricity. One of the most expensive actually. Still only 5 dollars a month on an old laptop for continual use. Basically if you actually care about the advancement of a peer to peer non centralized currency. You should probabily run one.
Not everyone can reasonably. But definitely more than a 1000 people from this reddit should be able to. It's hard ro believe 0.05 percent of this reddit couldn't become one. This is actually pretty important because without nodes from non corporate people. There's no way a peer to peer network will survive. It only takes 6 hours a day. You don't even have to run full time. Although it is better for the network if you do.