There are probably many reasons but I suppose the main one is that the majority of those holding bitcoin or altcoins are small holders which will need to use a great deal of their capital just to buy a hardware wallet, and as such it is not a very attractive offer for them, another reason is simply ignorance, whether that ignorance is about how to secure their computer or how easy it is for scammers to get to their coins they simply do not understand the risks and they prefer to not use hardware wallets.