There could have been a nameless Mook who went "Marco, can you save Ace?!", and then he goes "No good, his wounds are too intense".
You know, a little something.
You know, a little something.
I don't imagine he can just do that to people who aren't him, it'd be too broken and then Oda would also have to write an excuse as to why Marco didn't save Whitebeard or all of the fallen soldiers around him