They started putting the data cap in place in select areas I want to say about 5 years ago "as a test". I think they actually originally started with 250GB, but quickly found that was too low and raised it to 1TB. Then after about a year of select areas having the data cap, they changed it to all customers. So they picked that number 4 years ago, when 1TB probably was just fine for most people.
However, now I'm finding a lot of people I know hit the data cap every month. My neighbors have 3 teenage kids, all 5 people in the house stream netflix almost constantly when their home, and they are constantly going over the data cap. They were paying about $100 a month extra with overages until I told them about the option for unlimited.
I'd hope that Comcast realizes that 1TB isn't enough for a normal household these days and would raise it to 2TB, but I doubt that will happen. Comcast knows the only real alternative in our area is AT&T, and their service is both more expensive and worse. If you don't get the FTTH, the max speed you can get is 24Mbps. And if you do get the FTTH, it's expensive and unreliable and goes down constantly. So until there is a decent alternative to Comcast, their going to keep the data cap and rake in the cash from overages.
And Comcast uses the excuse that they have the data cap to stop the few people that use outrageous amounts of data. But there are much better ways to control that, so it is obvious they are just doing it for the extra money.
I just find it kind of funny how 10 years go, home internet was unlimited and mobile data was expensive as hell per GB. Now, they are practically giving away unlimited mobile data, every mobile plan with Verizon gives you unlimited data now for example, and most home ISPs are pushing for restrictive data caps...