Do they? Are they? Maybe you need to check if anyone else in this industry thinks this. If they don’t then you have to convince them that it’s a good idea, AND that your product is the solution to it.

You say “many of the vendors have APIs”; will switching to your product solve the problem of interfacing with them? Is this a problem anyone is having? Perhaps you can push adoption of your own API by making a product that talks to all these different closed APIs on the back end, and only requires its users to learn your API to talk with all of them.

Perhaps you should read the history of email. Wikipedia’s article on it starts by noting that it took fifty years for the current suite of smtp/pop/imap to become the standard. https://en.wikipedia.org/wiki/History_of_email