It’s not that it’s not happened in 50 years, it’s that it’s the anniversary of an event and it’s a big anniversary. Instead of a slightly raised security presence, it was markedly reduced. The US has heightened security every year on key holidays, and the US does’t have such an immediate threat right next door. Meanwhile Israel is generally more militaristic as a nation with mandatory service. The lack of defensive forces present on the day is just ludicrous.