My guess is he didn't write you because you raised a doubt in his mind as to what the speed limit was on that particular stretch of road.
Sadly enough cops don't know the speed limits posted on most roads. They don't usually obey them while on-duty, (responding to calls), or while off-duty, as they "think" they are above the law.
Sounds to me like your polite, respectful, and convincing argument paid off. Good job.
I know what was going through the officer's brain because I've made the same mistake he did, ONCE!! You can bet he won't make the same mistake again. If two on board, quite likely a training, FTO car.