Even when I was taken out to China with Samsung, they made us attend some 'lesser' events in addition to the good ones like athletics and the closing ceremony.
The problem is that people don't all want to go to the lesser events, and especially not the early stages. Later on, as Lord Coe said, the seats will probably be filled (but it was a bit misleading to say it's going to fix itself - as they must have known that early on people wouldn't bother).
There are a lot of factors;
1) As I said, not all events being that exciting so people waking up and going 'meh' if it wasn't an event they would have actually wanted to go to.
2) It was a weekend and many people given free tickets may not be bothered about going in their own time, if they can't actually go during work time. (I for example was taken by Samsung for my work, so didn't have to take leave to attend).
3) As mentioned on the TV tonight, many companies are now finding it hard to be seen to enjoy any perks, especially if they're laying off staff or risk falling foul of new tax laws.
So, with all of the above points and any I've missed, they probably shouldn't have given out so many free tickets for certain events - or had a strict policy where people had to turn up by a certain time or forfeit them, with others allowed in.