They'll put you on the next available flight - did you honestly expect them to bump people off to accommodate you?
Iberia do do a lot of standby listing - the fact it wasn't offered to you is most likely because it already had many passengers on standby for that flight. Would you have really preferred to hang around with uncertainty until past 8pm, then having to be rebooked at that time on whatever is left at that point in time? Or be rebooked from the onset and go to your hotel?
Not sure how you get to 24 hours from AGP to FRA, mind.