Answering the question in the title: it's easy - no one wants to spend their money on mining equipment that is constantly getting cheaper.
Another reason is - few people already have
A LOT of hashing power and they get an advantage compared to a regular folk. And regular folk is not happy with this fact at all
It is a complex question, but the main thing is this.
In POS, wealth becomes more concentrated at the top over time and there is no way to break your way into the economy without the 'permission' of existing stakeholders.
In POW, although mining costs can and usually become prohibitive, there is always an entry into the economy.