I think that early investors were almost purely for the idea. I'm investing in BTC because I'm sure that it will get to the $100K and even higher. I've been buying it at below $20,000 last year and now it's at $40,000+. Not a lot of assets are capable of that.
What triggers you to invest is just solely to make a lot of income from your bitcoin investment. People might have different opinion as to why they are investing that is different from yours but what matter the most to me is that; with my investment in bitcoin, I won’t be affected by inflation and the worth of my BTC will still be what it is even after being hit by volatility in the market.
Nobody should ever "invest" in Bitcoin because Bitcoin is not an investment, nor is it a way to get rich or make profit.
Sadly enough, the big number of adoption that have been received by bitcoin is because of the mindset of investment that they have about it. It has moved from its main purpose of creation to another alternate source of investment that can make them rich overtime.