Has this guest already left a review from a previous stay? If so, I would definitely not ask for repeated reviews.
Airbnb DOES harass guests for reviews - I was a recent guest and the number of requests for reviews was obnoxious. And truthfully, I didn't know what to write in one of the reviews (loved the actual host, but not so much the space) so I chose not to write a review.
It could be any number of reasons...
Has already written a review...
Doesn't particularly like writing reviews...
Doesn't know what to say in the review....
I have and will NEVER ask a guest for a review. Airbnb pesters them too much already.