Yeah, because they weren't sure if this was going to be a series or season finale until a week or so ago, they couldn't really incorporate the snap into the plot of the show. Imagine if it was the series finale, and the last thing you see is half the cast dying for no reason at the end, in an event completely unrelated to the plot of the show. And since the show wouldn't be coming back, there'd be no resolution to it.
That would be an awful way to end the series, and a terrible way to treat the characters in a send-off.