As some others have noted, there are so, so many factors when it comes to adaptations. The legal stuff itself is a huge hurdle, not to mention the financial aspect. People seem to think this stuff is easy and doesn’t take massive amounts of money and work. And, you know, time. Of course they want to make sure whatever story is big enough before they sink money into it (I’m not saying companies shouldn’t take some chances, because I’m tired of seeing the same old “we can make money off this well-known property for sure...and run it into the ground”, but I understand the risks involved, and why you can’t do this for every story).
I’m sure the more successful series are, and the more the platform grows, there will probably be change, but that’s more than likely pretty far in the distance. As creators, it’s our job to produce the best work we can, and grow our own audience and stories, and seize opportunities when they come.