You know, the only thing I can tell you is not to argue with the Pokemon way. What is...is. Prior to that, all Pokemon either evolved by trading, elemental stones, or level-up. Generation 2 was trying to make more creative ways for Pokemon to evolve. Why Golbat to Crobat...no clue. Perhaps a better choice would've been Gloom to Bellosom. Or maybe Swinub to Piloswine. Maybe Sentret to Ferret. But like I said before, what is is. Don't ponder it. It's like, "how does a Pokemon evolve by being traded with another trainer?". It makes really no sense, but that's just the way it is.