You get a call to drop everything and return and you don't put two and two together and think that IT is back?
Then you dramatically say you're leaving?
They could have left that out of the storyline. It also makes them look less commited to the purpose of the club.
Think of it as suppressed traumatic memories: the horror of what they faced as kids was so traumatising that their minds just did what they could to bury those bad memories, so they essentially couldn't remember specific details, but they knew that it was important enough to have to come back when summoned. So when they did come back to Derry, the memories slowly resurfaced, being among familiar childhood surroundings, and of course IT starts on them again, so they remember the full horror and terror of what happened and they really want to flee to the safety of their successful, safe lives, rather than risk fighting IT, as once was enough. That's my take on it, anyway.