Precise machining of both ends of the chambers is the reason. It provides perfectly round and smooth seating surfaces, as well as fixed chamber length. The screen seats snugly on the machined surface, so no particle larger than the screen opening can escape around the end of the screen.
Straight threads mean less torque is required to obtain a tight seal with proper gasket compression. Less torque is required to remove the retainer. The danger of "freezing in" is considerably less than with hard-to-break tapered pipe threads.
For 2-1/2" and 3" size strainers, the off-center drain permits nearly complete removal of liquid and dirt when blowing down the strainer. An added plus: Less liquid will spill when the screen retainer is removed.
Many screen materials, meshes and perforations are available.