If a flight may be cancelled - the pax will stay landside, Only once they are sure a flight will go - would you take through the process.
It is not just accepting the pax airside - as they can more readily be sent back but the luggage. the systems is designed to be one way! If a flight is cancelled - not just postponed - then all the bags have to be processaed as if the flight was going - but then manually returned AND you have to get the pax to an area where they can collect them. Depending on the destination, that may be airside.
Consequently, you keep Pax + Bags landside until you know.
As to the fault, having spent 27 years in telecommunications - and having known the man who was responsible for the (govt) BAA who ran the system - I have some guesses as to what went wrong.
Of course, systems go wrong and a contingency is needed but that has failed and any guess of mine revolves around Money and Outsourcing. but then, I'm a cynic and spent too long watching the process.