Why was it so hard to establish a "4th" kids network

https://animesuperhero.com/forums/threads/why-was-it-so-hard-to-establish-a-4th-kids-network.5738241/

Fox was the first to throw their hat into the ring, and through its Fox Kids Worldwide division, acquired The Family Channel from Pat Robertson. Rebranded as Fox Family Channel with a focus on targeting not just kids, but the whole Family. However, News Corporation failed to truly rock the children's cable world, the same way they rocked the Broadcast world with the FOX Broadcasting Company. Lackluster shows and conflicts between Fox and Hiam Saban led to the channel being gobbled up by Disney in 2001.

Next up was Discovery, who decided to go after a niche audience with Discovery Kids. Their strategy was effectively emulating Nickelodeon, but with an educational slant. Their main growth period was during the 2000s, where they were leased NBC's Saturday Morning block to use as essentially, glorified promotional space for the channel. This was effective, as by 2006, the channel reached 43 million homes. However, without NBC, the novelty of an edutainment channel for Kids quickly wore thin. So Discovery sold half the channel to Hasbro in 2009, and the two companies re-built it from scratch as the more entertainment based Hub Network, which showed cartoons and live action shows geared towards Families, many of which, were based on Hasbro's extensive line of Toys and Games. This also failed to hit mainstream, which forced Discovery to buy-back some of Hasbro's shares and run the channel themselves as "Discovery Family" which by some strange miracle, is still around today.

Now NBCUniversal is up to bat. After a series of acquisitions and buy-backs regarding its Sprout Pre-school channel, NBC decided they should have their own Kids network, and in 2017, rebranded Sprout as Universal Kids, named after its most important asset, the movie studio, Universal Pictures. Drawing from both Universal and NBC's catalog of shows, including the newly acquired DreamWorks Animation, Universal Kids once again is failing to truly be a threat to the big 3, who aren't doing too hot to begin with largely thanks to streaming services.

So why is it so hard for a new kids channel to become the "4th network" the same way FOX was able to break into the scene? Why is it still Viacom, Disney, and Warner that dominate the market?