It likely has something to do with cost and availability.
The side of a hifi box is usually longer horizontally than vertically.
Not sure if those heat sinks are extruded or milled, but either way the wider the heatsink (and the more fins) the more costly it will be to make per unit length. Also, there will likely be a lot more supply options available in narrower widths.
You are now thinking, yeah but the area is the same whether it is vertical or horizontal...
But if you use a narrower extrusion orientated horizontally the cut that you make on the extrusion is not as long, is easier to do, and you can probably get away with lower tolerances.
Also in horizontal orientation it is easier to hide the cut with the fascia and case rear. With vertical orientation you don't really want the case top to block the vertical channels, in which case the fins will be on show. This leaves sharp points that users can catch themselves on and it probably means more cut extrusions fail QC because just one fin got damaged during the cut.
All in all, I bet it ends up being a fair bit cheaper to have horizontal fins, and it still works ok ("kinda").
Edit to add... The amout of cooling required might only take up a fraction of the case side if the fins were vertical. However, that wouldn't look as neat and would require extra parts and fixings to achieve. If it allows you to make the whole case side as a single part and gives several times the area needed for cooling then having the slightly lower efficiency of horizontal orientation is a total non-issue.
