It's a combination of two factors.Can you post the stats on that because I’m pretty sure I have read differently. I could be wrong and have too much of a headache at the moment to go looking.
Vaccinated people are less likely to get it (so therefore less likely to pass it on) and all evidence right now is that their cases are milder and last for a shorter time, which means they have less time to pass it on.
Now, since Alpha Covid had a pretty heterogeneous tendency to spread (Re was 2-3, but it seemed to cluster in super spreaders) I'm not sure if Delta is more even or also very spiky in its transmission like that. It could be that the super spreader people stay super spreader if they get it, so the average transmissibility window being shorter doesn't help much. Or it could be that it is even MORE effective a that because it tends to prevent people from shifting into Super Spreader mode. I don't know how deep they've gotten into any kind of evidence on that.
But the basic core point is fairly straight forward --> Fewer people get it, so there is less transmission from them, and they get it weaker and for a shorter time, so there is less transmission from them.