Fluctuation , technical, fear of hacking etc. These are some common reason. It's need some time to make it mainstream
You highlight "fear of hacking". In speaking to people about crypto payments, often they ask about "hacking". The trouble is, none of these people understand what "hacking" means. They just hear the word in the news and know it is something bad and has something to do with technology.
Before crypto payments will see widespread use people need to,
- understand what it is
- how it works
- why they should trust it