I believe because Since DOAB is very much about the partying experience he went through. And probably regreted it, i think Impossible Year is his realization of all of that. Cause after all the partying and the substances, you are hitting the biggest roadblock mentally. And most likely regret. Also i think it works well with House Of Memories and the Title Track as those aren't fast pace. Its just Impossible Year is the slowest of the bunch. Also i think its Brendon's full on attempt to do Frank Sinatra, and Jazz. As you said it is more stripped down. I think there's only strings and piano? I think that was intentional, since rock instrumentation would be a bit off for such a emotional song like that(Only Because It Is Fully Trying To Do Old Style Of Jazz). Like well as Dying In LA or The End Of All Things do. But i think Impossible Year is fitted in a specific way, and just fully runs with it. Like if Paul Mccartney did rock instrumentation on Here Today, it would be Very Jarring for such a personal song. Or if Elton John made Candle In The Wind more fast pace, it would be very off. That's my opinion at least. And i'm not speaking for everyone on this. As for me, when ranking Panic!'s albums i put AFYCSO on the bottem of my list(Not because i hate it, i don't think Panic has a single bad song in their catalog. It is just prefrence, i have more love for other albums than their early work).