I can come up with three good reasons for such a meeting instead of a meeeting with all three democratic candidates;
1) Obama: You've done a good job Bernie making our girl Hillary look good and to run interference against the GOP. Now it's time to throw in the towel.
2) Obama: Hillary is about to be indicted and you're going to be the man. Congratulations!
3) Obama: Hillary is about to be indicted and I to know if you're going to support Joe or Elizabeth when the time comes. Being Vice President is not that bad to end a career on.
As for an indictment, it has to come sooner rather than later. If Hillary wins three or four primaries then it gets tricky and looks like Obama took her out over the wishes of the democratic party. However, if the indictments come before she can rack up some wins then it looks like Obama is being a supporter of justice. Hillary is not going to win Iowa, New Hampshire, or Nevada.