The biggest thing is that it's actually turned into the console industry. It's a mix of very well established games, and the new games that break out are those with fantastic production values, well received gameplay, and excellently run services.
Two recent-ish breakout hits are BanG Dream and Princess Connect, both of which are fully voiced games with hours upon hours of acting by popular actresses, nice looking art, and very aggressive update schedules. BanG Dream is also a notably well made rhythm game (I can't speak to Princess Connect's gameplay as it's not out in English).
You have a bunch of companies trying to put out new mobile games with 2014-2015 level content, game design, and production values into this market, and they keep failing, while their games that have had years of quality build up and audience investment keep succeeding.
Eventually they will figure out that they have to go really big at launch if they want new games to succeed like CyGamws does. Square Enix's full 3D monster/character Dragon Quest card game was notably their newest (and only directly published) successful mobile game last fiscal year.
Now, it's possible to get modest successes with less than that, but that doesn't move the needle at a company like Square or Namco.