The problem with bonfire on Motzei Shabbos is you have to prepare in advance. In the US you need time to get the police gates and fire trucks ready and have permits all set up. You also need to scour the area for flammable items (In case a piece of fire breaks off) you
In Israel they have to have the Police, Army and Fire Department set up the area. Comb the area for terrorists, Make sure there arent any flammable items nearby etc
Large crowds especially with fire just dont happen safely, you need to prepare. You need time to also get the fire extenguishers around etc.
Yes, if it was Sunday afternoon they could still work on Shabbos, but at least there is time to prepare. If the Bonfire is 1 hour after shabbos you just dont have the time to prepare safely