What would really stop a Suez Canal from being built

Would the Suez Canal only have been built once oil-powered shipping made it essential?

No. The trade and time-saving incentives were already strong; only a major geographical obstacle would plausibly prevent an earlier canal project.

The coal-to-oil transition affected the canal's later importance, while existing trade and time savings already supplied enough incentive for an earlier project. To prevent construction, an alternate history would need a physical barrier on a much more formidable scale than ordinary engineering difficulty. A Himalayan-scale range in the Suez region would also reshape the surrounding geography and constrain every alternative alignment. The question then shifts to whether engineers could cut a route across Africa or directly from the Gulf to the Mediterranean.