There are probably several reasons.
First off the manufacturer may have primary winding coils wound up for a whole family of transformers that use the same turns count and wire size. Secondaries are added as needed according to output specs.
Secondly it is probably always a simple calculation of wire total resistance versus total length. Primaries of the type of step-down transformers that you mention have many more turns of smaller gauge wire. Putting it near the core has a greater advantage for lesser total length and thus less total winding resistance. So it comes down to a simple trade off computation between secondary and primary.
I could imagine that there is a computation that involves wire size loss versus total winding length that resolves to the lowest physical size. The reason we see most step down transformers with the primary on the inside is because the computation most often wins out that way.
I look back to my early teen age years when I was very glad that old TV power transformers had secondary windings that I could easily remove and replace with my own to achieve the voltage and current ratings I wanted without having to touch the primary windings.