Because Kuma says:
"So you were alive, Roronoa"
"Thanks to your mercy (the RAW uses jihi which is mercy not benevolence)"
At this point both Zoro and Kuma know that Zoro made a deal and took pain for Luffy. There is no point to reiterate this fact. Zoro knows Kuma knows so there's no reason to say he gave mercy for letting him live. This would be what we call REDUNDANT.
Also the fact, because Kuma knows he let Zoro live and knows Zoro knows he let Zoro live, there would be no point to point this out. That would also be REDUNDANT.
Making Kuma being surprised or pointing out he's alive from the bubble itself after cutting it short and letting fate see if he'd survive or not.
Zoro replying to the bubble being cut short as 'mercy.'
This is called basic grammar and sentence structure. Or how conversations work.
"So you were alive, Roronoa"
"Thanks to your mercy (the RAW uses jihi which is mercy not benevolence)"
At this point both Zoro and Kuma know that Zoro made a deal and took pain for Luffy. There is no point to reiterate this fact. Zoro knows Kuma knows so there's no reason to say he gave mercy for letting him live. This would be what we call REDUNDANT.
Also the fact, because Kuma knows he let Zoro live and knows Zoro knows he let Zoro live, there would be no point to point this out. That would also be REDUNDANT.
Making Kuma being surprised or pointing out he's alive from the bubble itself after cutting it short and letting fate see if he'd survive or not.
Zoro replying to the bubble being cut short as 'mercy.'
This is called basic grammar and sentence structure. Or how conversations work.
If not then its only a headcanon