Why the prices for very popular items do not increase?
Some online-shop does practice price discrimination. 

However, the seller must play the price game carefully to maximize their profits: 


Lower price means lower profit per units but may increase revenue and increase overall profits
Higher price is more profitable but hurt sales
Shelter for the rainy day
Competition


In addition, influence factors such as inflation, hot money also come in play.  For example, during economy bubbles, people don't mind paying extra $100. However, once the market shrink, you will face a huge retract of revenue. Worst still, the manufacturers need to served bank debt because they borrow money to expand the business. 

Even items like EDM ticket may jack up the price(due to annual scarcity) , it may risk of losing patrons in bad time; too profitable, it will attract competitor enter the market.