Because of the new trend of "binge" watching. That's what we used to called "marathoning" a series.
When you think about it, the words used to describe it also aptly describes how people approach that now, compared to before.
Back in the day, marathoning a series was like a little endurance test. You usually wouldn't consume several episodes of a show in one go so you actually had to make an effort and set time aside to be able to do so, and if you did you were probably serious about getting caught up on previous seasons or rewatching a whole season because you wanted to be a fan. You also had to wait for a series to be done for quite a while before it came out on VRS or DVD.
NOWADAYS you "binge". A disgusting word in itself which implies you are just slobbering in the pig-trough of entertainment, consuming huge amounts of entertainment for no other reason than you've got too much time to waste. You're spoiled for choice and end up watching an entire season of something brand new while you turn off your brain. With no need to wait, new shit just keeps dumping into your lap and you don't even have to apply enough critical sense to seek it out because you WANT to see it. It just gets recommended to you, so you might as well.
Yes I can hear the screeching already, "hurr durr old thing good, new thing bad".
But it isn't about the shows, it's about how people consume them. I think this instant gratification with zero effort cheapens the work that was put into making a series more than a little bit.