Cylindrical shape in purpose to reduce the pressure.

P = F/A

Bigger area will reduce/absorb bigger Force to the material cage,and contrary

AS PER MY VIEW THE CYLINDER SHAPE DOES NOT HAVE ANY SHARP CORNERS OR EDGES SO "THIS REDUCES STRESS CONCENTRATION" 

It diverts the pressure which helps in absorbing it and no grain side is exposed 

Cylinders/Cylindrical shapes are best for gas containments because in a cylinder, there's an even and better pressure distribution because there are no Sharp corners or edges which leads to high stress concentration as a result of high forces. Thus the weak and break points on other shape containments are no found in cylinders.