If they were hiring an interior designer or painter who asked why a specific color or paint type was chosen, but gave no answer... that might still be weird.
In this metaphor the colour of the wall is equivalent to the product. The code is equivalent to whether the painters used brushes, pads or rollers or whether they used ladders or scaffolding - the customers do not care.
By asking why choose php you are asking why use rollers to paint.
If that's the case, asking about the choice of implementation language sounds like asking about the spec.
The end user does not care about the programming language.