Well, Sunday mornings are nice because it's an otherwise quiet time, good time to catch up on stuff that gets lost in the shuffle of real police work, like "showing the flag" on an issue that's not huge, but still a thing citizens want to see, traffic enforcement. Not a lot else going on, people can actually see that the Sheriff is out there enforcing the traffic laws. Doing this stuff during rush hour, that generates complaints, not kudos. And who wants that, really?
Have you been experiencing accidents on the stretch due to speeding? Have there been complaints about this to the Sheriff? Those are the things that might trigger enforcement actions. Otherwise, it's a thing that gets done when there is free time, like Sunday morning when the bad guys are sleeping it off.