Jan van Leyden
Adventurer
I believe your result comes from the typical player mind-set that encounters are a) dangerous things that the entire group has to be part of and b) not so dangerous that they will lose.
IME, any encounter that doesn't comply with those two concepts has to be clearly communicated multiple times in advance. And in as blunt and obvious way, often OOC, to ensure the players don't miss that *this* encounter is different.
Oh, my players are used to lose encounters and run for their life!

Sorry to hear that your players missed the note and the game didn't go quite as expected.
Oh, no problem with that. I was still tired when I re-visited this thread and posted my message. Playing til 1:30 am may have some effect on my mental abilities on the next day.