There are many many reasons for this. They can't all be explained simply.
1) Industry competition and hypercompetition might necessitate lower prices without an increase in profit - there might be an illusion of cheapness which isn't real
2) Horizontal integration - See Volkswagen Group, Vertical integration - See Apple
3) Mergers and Acquisitions to increase industry control
2 and 3 could both increase costs if they're employed inappropriately, see Hewlett Packard
4) Biggest reason - economies of scale - the more you produce the cheaper it gets per item. If you're buying steel in larger and larger quantities to make your product, the cost per kg (or whatever) decreases, etc
5) Experience Curve - the more you produce, the better you get at production - tacit knowledge of staff increases, mistakes decrease, efficiencies increase etc
6) Globalisation - see Li&Fung and US clothes industry
7) Legislative reasons - government grants for certain industries - government issuing more H1Bs etc
8) Industry lifecycles - as industries develop the dynamics between companies change. In a maturing industry, prices may decrease due to competition. In the late stage of an industry lifecycle the opposite happens as competition decreases
9) Product lifecycles - more people buy products as the lifecycle of the product matures. If the cost of goods sold doesn't scale linearly with production (e.g. selling a program) then the price can be decreased. The opposite happens as a product's life declines
10) Production efficiencies
11) Control of the supply chain - as you sell more you have more control of the supply chain and have advantages over competition - see Dell during their heyday
The list goes on and on.
You'd be best off doing a PESTEL analysis for the particular industry that you're interested in, and then if you're really interested, read intros to Organisational Strategy, Marketing, Operations Management, Supply Chain Management, Finance