I'm not sure if there's a 'source' for this question but if someone else wants to answer after me and give statements or whatever then you put more effort into this than me :P.
A few things to remember, firstly Gen 1 was hectic and unbalanced, pretty much completely. Entire, huge mechanics that are now heavily associated with the game didn't exist yet. I'm guessing the rate that you picked up EXP wasn't huge concern for Game Freak at the time.
However, the games became more and more balanced (arguable) as the times went on, and Game Freak put more consideration into the flow of the game, and how quickly you progressed. They wanted it to be more difficult, or require you to grind longer and work harder if you wanted an easier fight. This is speculation but a lot of the NPCs who give you the EXP share mention that it's 'good for raising weak Pokemon', which I'm guessing was their main intention at this point - to allow you to just train and practice while levelling up super under levelled Pokemon so that they could catch up.
What changed in Gen 6? Game Freak / The Pokemon Company wanted to shift to a more competitively oriented game. They wanted more people to compete online and to take part in events like Nationals and Classics. The EXP system supports this, by allowing trainers to level up and try out more Pokemon within a reasonable amount of time compared to training 2 at a time. This is also why they buffed certain breeding items like the Destiny Knot.