This is my hypothesis only. Would like if somebody confirms this.
So Tesla regardless of being valued at 50-60 billion dollars back until 2018 wasn't part of SP500 since it hadn't posted 4 quarters of profits. Ocassional quarter profits, but no consequetive profits.
That changed in 2019, when the company posted 4 consequetive years worth of profit.
After successful profit years, even if minimal, Money Managers at Wall Street took a note and started aggressively buying to push the stock up. So much so, that even before the inclusion of stock in index, it was at 400 billion dollars worth of market cap. And passive indexes were forced to buy in the stock at rebalancing.
I don't have data to back the initial rally between end of 4th quarter profit and date of sp500 inclusion. And this where I'm guessing. Perhaps y'all could correct me or show some data if that's actually true.