Why did the delegates to the Syrian General Congress object to the plan to create mandates in the former....?

they didn't want to be ruled by Europeans. The collapsed Ottoman Empire was carved up between the British and the French to rule as mandates until these areas were ready to govern themselves; what is now Israel, Palestine, Jordan and Iraq went to the British, and Syria and Lebanon went to the French. The Syrians wouldn't be very happy about this as they had only just thrown off the yoke of one foreign empire.