Don't the threads cut in about 1mm? So your 3mm thick wall would effectively have about a 2mm sheer strength (including multiple stress risers).
Many of the vintage steer tubes also were either slotted or had a flat spot for a keyed washer, creating a weak spot for an internal expansion stem.
The compression force on an external stem is likely more stable.
An 1 1/8" steer tube would also be made with minimal additional material, but have added strength (plus it is easier to source parts for without resorting to shims).