3. Fluctuating value: The value of bitcoin isn't steady as it rises and falls. The uncertainty in the price and value greatly hinders the general adoption worldwide.
4. Awareness: Nearly not enough effort on creating and spreading positive awareness on it as some government and individuals alike is hell bent on spreading negativity around and poisoning the minds of prospective users. People need to be aware and understand what bitcoin is really about and be awake to its uses.
Those two points are a major cause in my opinion. I would also add that buying a bitcoin is not exactly a straight forwarding thing to do since usually you have to open an account on an exchange, send the documents, wait to be verified, transfer the money and then decide where to store them. Many times when you explain to people all these steps, a lot of them immediately think it's too much effort.
That could be one on the many reasons, because people are usually kept their hands off to complex things such as creating a custodial wallet or exchange wallet as new crypto investors would always resort to such options as they see a lot of advertisement all over the social media.
But there is actually a few steps to purchase your own Bitcoin without using those services.
But then again, people will always have the first impression of purchasing Bitcoin to an exchange.