The right side leak is observed by indication of the right tank. It is entirely possible that the leak point is not in the tank itself but it in the feed line of engine no. 2. Opening the crossfeed could expose the left side fuel to the leak point resulting in unnecessary loss. Even if the right wing tank had been ruptured you still would have loss of left wing fuel through return flow via outer to inner right tank and then out - with open crossfeed.
In order to prevent these the Xfeed is kept closed and engine no 2 will starve out once right side fuel depletes through consumption and leak. You get to keep all available left side fuel for eng no 1 in the ensuing OEI case.