At the risk of sounding flippant, because the FCTM and MEL say so!
I imagine it's to cater for mis-selection of the wrong reverser if one is out. The guidance says select both for RTO and landing.
I'm always curious about these instructions by a manufacturer. This scenario is not a special technical difference to the type. It is a case of airmanship and judgement. I can not see what it has to do with an Airbus v Boeing v another type. If that is the case then I can not see the authority with which a single manufacturer makes that an FCTM issue.
If someone can enlighten me with sound reasoning I am open to discussion.
What I have seen, in the sim, is a pilot, in an already non-normal situation, who is a little nervous, try to select both reversers and become confused and concentrate on the blocked reverser and not focus 100% on completing what up till then was a successful landing.
IMHO, if in doubt, it is less risky to rely on TR's and just use auto brakes of size 11's.