Because, and this is shockling news, you assume that there is space and people are just getting closer together and that if there was a problem someone with better information than you have would be passing it on (for example a police officer or steward with a load hailer, or someone in the control room using a PA system).
There is a whole field of scientific study devoted to modelling crowd dynamics and one of the things they learned quite early on is that what seems to be an insignificant factor at one point (IE people pushing forward like they do in trains, crowded supermarkets etc), can have catastrophic effects out of sight, this disaster as well as various completely avoidable deaths when things like fire alarms have gone off in large buildings led to people starting to study what exactly happens..
To the people at the back it's often no different to the sort of crowd conditions you are used to, the people in the middle may be aware something is wrong but not sure what or how bad, and have no way stop it, and even those just a few rows back from the front may not realise how bad it is where people are being pushed against something that doesn't move/give like the human body.
To put it simply you're wrong in your assumption that people entering the stadium must have known something was seriously wrong.