I think the main reason is the lack of funds. After all, if you have enough money, you will not be afraid of losing it because of the fear of a future price drop. You just try it!
There are also many people who don't hold bitcoin, simply because they don't have enough knowledge. They just don't understand what bitcoin is and why it is needed.
Well even if people do have enough money, lack of knowledge would still arise or let's say.....misconception. Just like I mentioned in here, I had the wrong thoughts about Bitcoin back in the days and yeah I kinda regret that I let that thought went through for years causing me to miss out a lot of $$$$ to sell. Imagine if like around 2013 you had bought a whole Bitcoin and you finally held til late 2017 which was like a memorable moment for me in my whole crypto life even if we already had a higher ATH.
Honestly, I would love to have more money to invest with more bitcoin, but I think the ability to balance my life with such an investment is right for me. I understand the potential and future of bitcoin to increase in price and possibly hit a new ATH, but I need to live out my daily expenses, I still have my own crypto investment goals, there are periods where I need to earn money to be able to buy bitcoin later and wait for it to increase to make a profit. After accompanying the market for a number of different cycles, I realized that this place really offers a great opportunity to make people's lives less difficult.
Well Bitcoin ain't really going to make you rich. Don't depend too much in investment making you rich solely by itself. Having another stream of income is the secret, but finding that "another stream of income" is the hardest thing to find.