Everyone is speculating about why RR and/or Boeing didn't report the ACARS messages right away. Please consider the likely possibility that they did report it right away to their national security services in the UK and/or US. When these security services were notified, they must have considered the very real scenario that a plane has gone completely missing and has flown for several hours without any communication. Their primary interests at that point are going to be to quietly ascertain as much information as possible about that plane and its intentions/possibilities/potential whereabouts and act accordingly. They would want to do so prior to telling the world that they have acquired all this data.
I bet if the ACARS showed a normal looking descent or shut down event (i.e. a landing), we won't know about it for some time. It leaves the baddies thinking we still don't know if they survived or where they might be. Not to mention telling the world there's a plane in rogue hands could spark false alarms and panic.