Answer: The rivalry of these two news providers has increased due to INCREASED MARKET COMMONALITY.
Explanation: Market commonality can be defined as the significance and number of markets where a business and their competitors are actively involved. A business and a competitor having the same characteristics and attributes of customers is referred to as market commonality. Here, competitors are vying for the same customers and how many markets they share in common.
Here, Wall Street Journal's moving from financial news reporting to various other types of news means the two newspapers are now competing for the same customers.