[ConservativeMind]

Easier to prove?

It averages out okay, but a single large dose can be handled in a way by the body that negates some of the benefit.

[ProtectOurFreedom]

I did some research and it appears the main reason is people are much more likely to forget to take a daily dose than a monthly dose.

Monthly vitamin D dosing is better than daily dosing for children and elderly (more likely to be taken) – June 2017 Vitamin D every 25 days may be BETTER than daily – RCT May 2018
A Randomized Study to Compare a Monthly to a Daily Administration of Vitamin D3 Supplementation
Nutrients 2018,10, 659; doi:10.3390/nu10060659, 23 May 2018

The half-life of Vit D in the body seems to be around three weeks, so the monthly dosing is going to cause big fluctuations in Vit D serum levels throughout the month.

Half-life of vitamin D varies

This "VitaD.org" site feels a bit odd and I cannot find any information about who is putting it out. I saw some mention of Pakistan when looking at the various articles. But they sure are collecting lots of data from many studies and are organizing it fairly well.

I started daily Vit D early in the COVID pandemic. I use a "Pharmadose" pill organizer which I load with 28 days of Vit D, BP meds, and statin so I don't miss a day (I do not take different meds at Morning, Noon, Evening and Bedtime, so I can get 28 days of meds in the box).