Or maybe they didn’t want to waste time on a candidate with such superficial focuses? Next time ask them why they chose to paint the wall dark blue instead of navy blue.
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.