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.
One more thing. The primary reason for isolators is to prevent signals from being coupled down the antenna and back into the power amplifier and causing intermodulation products. This comes out in a combiner spec as Transmitter to Antenna isolation. The isolator is designed to circulate reflected or coupled power away from the amplifier source and to a dummy load to be dissipated as heat. The isolator can and does dissipate reflected power, and it is designed to do this, but it is not primarily designed to be a heat management device.