But back in those days players were used to be left to their own sense of orientation… Many excellent, complex games like Blaster Master for example, were sorely lacking a map (and a save feature in that particular case)
As you point out, (and I hadn´t noticed, even though I surely benefited from it) the producers of Metroid 2 even went the extra mile to put unique enemies as a sort of landmark to help with this problem, instead of providing a mapping device. Maybe this apparent no in-game map policy was a conspiracy to boost the sales of Nintendo Power?

Luckily three years later the sequel would address that issue in such a trend setting way!

Also, I agree with Jama in the comment above about the map. It wasn´t really necessary to include such a feature in a game where backtracking is not only not required, but almost discouraged. But the most important element that would be hampered by an automap would be the feeling of being lost in the lonely and dark bowels of an hostile but almost completely deserted alien planet.

This game relies so much in the lingering feeling of dread it can leave on the player with its atmosphere, its sounds, its overall creepiness, that it must have been the designers deliberate claim to fame for this project. This isn´t at all the most fun to play game in the Game Boy library, but those of us who completed it in our childhood on the old green screen will never forget it, even if the desire to play it again, even after 20 years, is non existent, at least in my case. But it made an experience for the ages.

As they said in the Gamepro review that convinced me to get the game, “No other Game Boy cart can grab and hold your imagination like Metroid 2” And at that moment in time that was absolutely true.