In the first century, the disciples saw all the parts of Jesus’s prophecy being fulfilled and realized it was the sign he told them as they saw all those things occur whereas to everybody else of that wicked generation it wasn’t a sign nor did they realize it was nor concur with the apostles that it was. They rejected the preaching work and neither did they flee Jerusalem when they saw the Roman armies or believe it was a sign. To that evil generation the sign that Jesus gave to his disciples wasn’t a sign for them but it was for his disciples.
The sign that we @ JW have been preaching about all these years too although experienced by and affecting everyone like in the first century is not viewed as the sign of the last days either by people in general.