Because this game is made in a C64 high-res mode which is very similar to ZX Spectrum graphics. It meant that each 8x8px square could have only two colors, background (black in this case just like in most games made in this graphic mode) and second color which could be anything. C64 games like Monty on the Run and Spellbound are also good examples of similar graphics but just like Saboteur 2 they were actually made initially for ZX Spectrum. High-res mode was indeed not very popular on Commodore64 and used mainly for Spectrum ports, most games made originally for C64 used multicolor mode with graphics and colors you refer as better. It allowed to use 3 colors per 8x8px square plus background color but in exchange all pixels were stretched to double width, that's why high-res is called high-ress. It IS possible to make Saboteur 2 better but it would require to do all the graphics from scratch, obviously people prefer to just port it with minimal changes and minimal effort possible.