Only does people don't have enough knowledge about it will doubt on Bitcoin. But if they came into to do further more research and know those things then provably that investing with bitcoin for long term would really be not a problem with them.
There are lots of sources where they can learn lots of information about bitcoin and there's no excuse for people to research more deeper information about it. But if they still choose to be doubtful on this asset and believe on traditional people saying that this is scam then they are wasting opportunities what bitcoin could potentially grab. Bitcoin is strong asset now and hopefully this condition together with those good events also acceptance happen is strong basis to think about that bitcoin is not easy to collapsed asset. Those experience bitcoin investors would provably continue what they are doing and we see people doing fine and provably would earn more from it in future.
It's understandable but not having enough knowledge is not enough reason for someone to doubt something that they've seen increased overtime. That's why their's a saying that (people perish because of ignorance) and people who fall under that category don't know what they're missing out. Most people who are yet to start investing in Bitcoin are just so scared of the dip and just like you said they should've made research about it to know better and understand it before condemning or ignoring it due to fear (something that's very beneficial on the long run) and that's why I feel it's not only ignorance but unwillingness to learn, procrastination and so on. Well whether they still doubt Bitcoin or not, it's not going to change the fact that Bitcoin must attain greater heights it's already on it's way to 100k and that's enough reason for any one to jump in and start hoarding. Like I said earlier, they don't know what they're missing and I bet that when Bitcoin gets to 100k their excuse would be that the price is too much for investment, If only they knew.