That and maybe not needing to work out and/or dehydrate himself to be buff as hell for shots, if the body-covering yellow suit is anything to go off of.
If he doesn't have a topless/nude scene with Reynolds touching his chest for comedic effect, I'll be kind of disappointed. But really, I think he's doing this for a mixture of the tremendous Disney money and because he's friends with Reynolds.
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.
My theory is that he's not actually playing Wolverine and him being in this movie is a big gag, like he's actually playing Deadpool's dad or some other character. I don't think any of the official announcements actually said anything other than Hughes Jackman is in it