Because, they always kept saying that (as far as the second coming goes?) he would come "like a thief in the night." They stressed that point many times. A thief in the night. But they also said, he would come with a sword, and vengeance.
So I thought - something doesn't line up here. Here's what I think might have happened:
Prophet A: "Hey, bud, come here check out this vision I'm having."
Prophet B: "Hey man, that's a wild vision. Is that the Lord?"
Prophet A: "Clearly, it is the Lord."
Prophet B: "So he's coming back?"
Prophet A: "He is! Joyous news - but I need a little help describing what's going on here, in this vision. I'm not sure how to describe that."
Prophet B: "Well. Ok. Sure. Let's watch."
They watch the whole vision.
Prophet A: "OK. So, let's start from the start. He's dressed all in black."
Prophet B: "I didn't even see him at first!"
Prophet A: "Me either! It was a shock and a surprise when I did see him."
Prophet B: "Caught me unprepared."
Prophet A: "Clearly his coming will be stealthy."
Prophet B: "Like a 'thief in the night'!"
Prophet A: "Well, no, not really, I mean - that's not exactly the word for it. I wish we had a word in our culture for what we just saw! He crept in through the window..."
Prophet B: "Like a thief in the night!"
Prophet A: "...with a sword and with vengeance, and then - and then - "
Prophet B: "To be honest, I couldn't really take it all in. What were those sharp pieces of metal he kept throwing around?"
Prophet A: "Exactly! And that swinging roped-clubs arrangement - so fast! And he fought...dancing."
Prophet B: "It was like dancing. I don't know. I think we need to leave all of that out. That's not going to translate for people. Just go with 'like a thief in the night'!"
Prophet A: "Ahhh, you and your thief in the night."
But in the end, that was what they went with. They didn't have a better word.