Minions are highly addicted creatures who are hopped up on...

Gold

Think about it, why else would near defenseless little creatures scurry across a battlefield to take down towers and kill eachother?

why is gold the only thing minions end up dropping when they die?

The people behind the league before the retcon captured creatures, got them addicted to a substance that highly effects their senses and brain washed them into fighting just for people's entertainment.

Why do you think they got rid of purple minions?

two reasons

1 the purple minions are now aware and purely fight for their freedom (which is why they are in the other game modes nobody really cares about so nobody can find out the truth.)

2 the entire purple minion race is rare and dying out.