1. They invest in a Ponzi and their company runs away with their money.
2. They use some free wallet and again the company run away with their money.
3. They install some trojan through a program they're unaware of and the hacker steals their bitcoins.
There could be many possibilities but I thought it's better to post easy-to-understand ones.
I agree with all three points, but I believe that the no.1 reason is carelessness. I have heard about numerous instances when newbies lost their coins, due to careless handling. In some cases, they purchase a hardware wallet and store their coins in it. They forget to keep backup of this wallet and if the wallet is lost, then all the coins are gone. In other cases, they never bother to activate 2FA or email confirmation in their online wallet. Such wallets are like easy pickings for the hackers and crypto thieves.
Carelessness due to lack of knowledge, It's tough to see those newcomers who losses their money.
Forget about the wrong impression that this market is a place where it's an easy way to earn a decent amount of money.
Indeed, this is a good venue to invest in and have that equal chance to make some to huge amount of profits, but always
remember that it's your knowledge that will lead you to succeed.