Why Gas cylinders and all other high pressure vessels are cylindrical in shape .....???????

Gas cylinders are made in cylindrical shapes , although spherical shape is twice the strength of cylindrical pressure vessel.

But spherical shape cylinders are not preferred because of two main 
reasons  :--

(1.) complex manufacturing procedures
(2.) difficult to transport

Because gas cylinders contain high internal pressure so we cannot use shapes like rectangle and cube  , because there will be many stresses associated with such shapes.

While in case of cylinders , only hoop and longitudinal stresses are present.

Rectangular and cube designs will be more prone to failure at corners due to bending stresses and welding failures.

Now this is why gas cylinders are made cylindrical in shape rather than spherical , cuboidal and rectangular.