Why do rangers get two-weapon fighting in second edition AD&D (and later editions)? Well, I’ve heard reasons that claim to be informed by people from TSR, but this is why it made sense to me at the time.
When I looked at the ranger class, I primarily saw Robin Hood. (And perhaps the strongest image of Robin Hood for me then was the Disney movie starring anthropomorphic animals.) My image was of someone who wouldn’t wear heavy armor or carry a shield. Yet, under the AD&D rules (as I and my friends played or misplayed them) the advantage of heavy armor and a shield were too good to pass up. Giving the ranger class two-weapon fighting ability and restricting it to when they were wearing lighter armor gave mechanical reasons for our rangers to look more like my image of the class.
My opinions have changed these days. I have different opinions about armor. I have different opinions about two-weapon fighting. I have different opinions about the ranger class. At the time, however, it made perfect sense to me.