Obama doesn't want to appear weak.
I mean, wasn't that one of the factors in picking Biden?
That has nothing to do with weakness. It's probably more about consistence.
Obama could be sending a clear message about where he stands on Palestinian / Israeli relations.
In any event, not having a former President speak at the convention will probably become routine.
I can see many Republican conventions in the years to come taking place without GWB.
Look how many Republican candidates in the mid terms didn't even want him campaigning for them