The 19 Inch Rack Standard. If we don't have enough isolation, the amplifiers will produce intermodulation. If we use cavities which are too large, or if we use too many cavities, the combiner won't fit in the rack or it will take up too much space. And then, on top of all of this we have to deal with each other in a competitive situation. This makes a pretty nice cavity and the pipe is less costly than having a special extrusion made.
Square cavities stack better than round.
Because its more expensive to TIG weld aluminum plates together to make a square cavity than it is to buy round extrusion and chop it off. 15 dB of isolation through the cavities at 100 watts at reasonably low insertion losses. A 90 degree hybrid will cost 3 dB of loss for any two channels coupled. 3.2 dB is a good figure for modern hybrid couplers. Hybrids have some advantages.