Although the reasons behind the non-inclusion of Schneider in the film are unclear, it is thought to be about money issues. In a radio interview last year, Schneider himself cited money issues and a scheduling conflict with a CBS sitcom he was doing. However, the scheduling conflict just doesn't add up because at the time they were shooting Grown Ups 2, the sitcom had already been cancelled. See the details on splitsider.com/2013/03/nick-swardson-is-taking-rob-schneiders-place-in-grown-ups-2/.
If you watch one of the Howard Stern interviews with David Spade and Chris Rock promoting grown ups 1, they mentioned that Rob Schneider was the only one who haggled for more money. The problem with a movie that has so many big name actors is that they definitely won't be making as much as they normally would initially, but I know Sandler, Spade, Rock, and James get a piece of the back end. Most likely Rob Schneider turned it down because there was no room for him to haggle.