They could have used a graphics mode, yes. But using regular 80x25 text mode made the game more accessible in terms of required hardware (both graphics hardware and processing power, especially with a "slow" interpreted language like QBasic), which could arguably have been an important factor for writing this kind of demo program. 80x25 text mode was available on all graphics cards including old CGA cards. There was no comparable graphics mode on CGA with the same resolution and number of colors. Their other demo game - GORILLAS.BAS - used EGA graphics instead.