I wanted to go to this game but couldn't because the tickets have been sold out for weeks. In fact every league game has sold out for about the last five years.
The empty seats aren't because tickets didn't sell, they're season ticket holders who didn't turn up. The thing about City is that they still have a very working-class fanbase and the part of the city where the club is based is one of the poorest areas in the country. Many of the ones who go to games barely have the money to keep their families fed and clothed, and every season their season tickets get more expensive. When you factor in the cost of the travel, the beer and food you have to buy when you're there etc, and then factor in that you've probably got a couple of kids coming with you too so you can triple those costs, then even if you've got your ticket guaranteed sometimes you simply can't afford to go. It's especially bad for cup games, when the season ticket doesn't get you a seat - it makes it hard to turn up each week. Only about 5 years ago, tickets were far cheaper and City only played about 20 home games a season. Now they can easily play 30 of them. People just can't afford it any more.
As for the seats emptying at the end - it's regrettable, but people flock to the exits early in order to get to their cars early. The roads around the Etihad are packed after the games, and on top of that you've got about 20,000 fans walking out who are all trying to cross the roads at once. The streets become gridlocked - it can easily take an hour just to get out of the car park. Some people just decide that the better of the two evils is to leave 5 minutes before the end.