Prior to mass-production, cars were very expensive and only the rich could purchase them. Why did mass-production of the car allow everyone, not just the rich, to purchase it? Mass-production increased the supply of cars and the price of cars dropped as a result.
Less expensive cars were made and therefore they became cheaper.
The middle class demanded a cheaper car, so Ford mass-produced cars.
The assembly slowed down the production of cars which made them more expensive.