I'm guessing you were a regular supporter standing on terraces back then Robbo? How could somebody with 1000's of people in front of them and being used to being packed into pens so tightly that they couldn't move, know the pen they were trying to get into was full? Was the pen blocked off? Were police/stewards there to tell them it was full? Were they directed to the outer pens?
Quite simply because you can, when it takes a great effort to shove the people infront, its OBVIOUS that there isn't enough room.
I've been in terraces loads, I don't think I've once ended up standing where I first stood, as the crowds come in everyone moves further in, I've been at the back, front middle. Its VERY obvious when an area is full, its VERY obvious when there is loads of space left, when you have to very forcefully shove the people infront of you..... there is no space... it really is that simple.
LIke I said, people packed in like sardines in thousands of terraces for decades with barely a problem, yet this one time fans were incapable of seeing when an area was full, and just ploughed on in anyway?
Think about how tightly packed people were to be crushed, then consider exactly how much force the people at the back had to be using to make more space and push them even tighter, this isn't a small amount of force, and yes, common sense absolutely dictates when there is space and when there isn't.
No one has said they did it on purpose, no one said they intended to harm anyone, but when people get stupid and ignore common sense people get hurt.
You've been to non terraced games, can you not tell when you come out of the tunnel how busy an area is? When you come down the stairs to a platform can you not see how busy it is, do you judge a platform to be so busy you stand still and wait, or not that busy and you can dodge and weave your way further down? Is this not really quite easy, yeah, every terraced game I've ever been to its easy to tell when its packed. At any one of these times, any single one, you or I could simply push as hard as we can and push people forward.... I choose not to, you do, most people do.