The number of engines being used in the FH, being much greater than Delta IV Heavy. The ignition process is complex for a setup and plumbing involved with that.

The number of engines being used in the FH, being much greater than Delta IV Heavy. The ignition process is complex for a setup and plumbing involved with that. And unknown interactions such as vibrations, etc. that they have yet to see from firing 27 engines at once as opposed to 9.