A bomb on one plane at SSH could be on any plane - could have been a random act (ie to whichever plane the bomber could get access to)
The downed aircraft was at Sharm overnight - a lot easier to gain access than a typical short turnaround by a LoCo.
'Bombproof' security of pax luggage wouldn't prevent this.
Stable door, horse, bolted.
As has been said - the terrorists only have to get lucky
once . . .