I think it's this.
Flying high is efficient because of vastly reduced drag and its corollary - increased speed. Increased speed attracts passengers, makes better use of your capital assets and your crews. So you go as high as you reasonably can. Jets can generate the power required so design them for high altitude.
Against that however you have complications such as exposure to sudden decompression and falling speed of sound.
These factors combine to give you an optimal flight zone of 30-40000 feet.
Note that if you change certain parameters eg the cost of fuel rises much more than the cost of crew or maintenance, then this optimal level may change radically eg see the return of propellors or engines optimised for lower altitudes.