I guess they had talked about doing this movie awhile back but it wasn't really possible at the time with everything going on with the rights to movies/characters. They never really thought this would have been possible, but once Marvel took control over everything, it became a possibility and Ryan went to Hugh with the idea. He was a little reticent on doing it because he thought Logan was the perfect ending for character, but they said that it wouldn't alter the timeline of what happened in the movie so he agreed to do it.

I guess Reynolds and Jackman are buddies too which probably made the convincing a lot easier.

That and the dump truck full of cash that @winebaths referenced probably didn't hurt.