You’ve hit the nail on the head when you say that ‘modern machines do not suffer with memory shortage...’ - this is being written for the ZX Spectrum, a machine that’s 39 years old, and has only 128K of usable memory. It also has next to no hardware support, so that memory is for all the code, graphics, sprite routines, music, SFX, etc. In order to get the most out of the machine and this memory the game is written entirely using Z80 machine code, and compression is the only way it will fit in memory.