Because the new format is easier for their (new) users but removing the old one would be a pita for their (existing) users.
You can't get things 100% right all the times so you have to decide what to do when you realize there is a better way. You have a few choices:
a. You don't improve the interface and instead you offset the issue by adding more documentation, helper scripts, templates/examples etc.
b. You improve the interface, possibly in a backwards compatible way.
So, (b) presents some maintenance costs, but (a) too has some costs! Some organizations tend to prefer (a) because it's easier to just make it somebody else's problem (e.g. some other team will do documentation, some other team will do the developer advocacy).
So it's a locally optimal solution and hence it's often chosen despite (b) being a globally optimal solution, since the product will be easier to use.