This seems to be mostly a US problem, in other Europe most of the time clear lamps are tubular and diffused lamps are elliptical. However particularly with low wattage and retrofit lamps, this does not seem to be the case in the US.

For example, most clear 360W HPS retrofit lamps are BT-37 shaped, which matches their MV counterparts. But why not make them tubular (like normal 400W HPS)? The envelope is clear anyway, making it bulb shaped doesn't make a difference.

And for smaller regular HPS wattages this is still the case, most 35-150W HPS lamps are not tubular in the US. Often they are BT, E, or ED shape. Is it to reduce the surface temperature of the envelope?

And of course out larger wattages (250-1000W) are almost never actually tubular but rather a strange elongated elliptical-ish shape. How did this shape come about? Why not just use plain tubular like the rest of the world?

Now I know for things like MV and MH (which are usually nitrogen filled in the outer envelope), having a tubular envelope can sometimes restrict their use to vertical only so the envelope doesn't end up melting. But I haven't heard of this happening with HPS before.

Anybody have any ideas?

Mechanical robustness against the external atmospheric pressure?

The most robust shape is a sphere, the least (assume smooth surface now) is flat panel. The curvature helps to make sure no part of the material is exposed to tension and no leverage amplifies any of the stresses.
These lamps use rather long internal structures, so true ball becomes impractically big.
The tubular has curvature only along one axis but becomes rather slim shape along the lamp working structures (the arctube with its frame and connection), elyptical across both direction, so it is stronger. Plus has its glass further away from the hottest part of the inner workings, so does not need to work at that high temperatures and that high temperature gradients.

On the other end the tubular means it could start from a tube - an easier format to make for a high volume production, which may make it cheaper even when eventually using a bit more material or requiring a bit more consistent structure. Elyptical requires more forming, which may make it more expensive for processing and more prone to defects that make it weaker.

At the end both are an engineering compromises, neither a "clear winner", their exact choice was dictated by the weaknesses and strengths of the particular plants and processes they originated from, but to big extend also the legacy - continue making what has been working and what people were used to in that area, e.g. also what the various fixtures became designed for (lamp anchoring mechanisms,...) and what the lamp engineers were familiar to work with (so they know what they can get away with and what not to do in their designs when using given shape, so avoid mistakes).