I'd guess the reason for the lack of retail investments to be due to the last times bull run, where most people only thought BTC could go up and where then burned when it dropped from $20,000 to $5,000 in a month.
It's not true that Bitcoin is drop from $20k to $5k in a month as you say, first time price is go so low was in
November 2018, and from December 17 when we have ATH it is hold above $15k until January 7, 2018 so anyone who know anything about the crypto market had enough time to make a safe exit.
I agree that big money was probably the cause of Bitcoin jump from $3000 to $14 000 in just few months, but it was some kind of test since they stop at one point. Even if they buying with help of OTC, when buying large quantities of something that has a limited supply, it must have an impact on the price. Maybe they change strategy, buying smaller quantities ensures a stable /lower price, and they have it now.