Unfortunately, pure soap isn’t actually a very good cleaner, and the oils your body naturally produces and general worldly grease, in the case of eVent stop the actual membrane from doing its job* and being completely waterproof. In general these oils will also reduce the ability of the DWR coating to attach to the outer fabric.
The only way to clear off the oils your body produce is to use a detergent, and the residue this leaves behind can then be washed away with pure soap, hence the recommendation to do two washes before re-proofing.
*eVent is more breathable, because the the oleophobic “layer” is actually a coating within the expanded polymer matrix that makes up the waterproofing layer. If this gets contaminated by a buildup of oils, it will allow water to pass through the membrane, and so not be waterproof