Simple, they have hit a price range that the average person feels they can spend and not feel to bad about wasting money later if it sits on the shelf.
You would probably need to edit your question to "Why do you think millions of people bought an [Amazon] Echo Dot or a Google Home [Mini]" The evidence is clear both are the lowest costing home assistant device for their respective lines. At $30 on sales this season it could make it even into many gift swaps with that budget. Just as a few years ago the Firestick and Chromecast did.
You would probably need to edit your question to "Why do you think millions of people bought an [Amazon] Echo Dot or a Google Home [Mini]" The evidence is clear both are the lowest costing home assistant device for their respective lines. At $30 on sales this season it could make it even into many gift swaps with that budget. Just as a few years ago the Firestick and Chromecast did.