Pretty simple (and good practice), in that the longer they take to post the points then the more likely you are to spend more on the card, plus they'd probably factor in some people also having a balance and so getting that very profitable credit card interest.
I'm a bit surprised that you are surprised. Banks don't give big bonus in the hope you churn, they actually want you to use the card. Short turnaround rewards churning so no surprise to me the banks have increased the waiting period.Just baffles me why they drag it out.