Why are lunar calendars not more widely used today

The lunar calendar offers no advantage or benefit, compared to the one that

we're using.

The moon goes through its complete set of phases 12.368 times in one year.

So if you're going to flip your calendar to a new month each Full Moon, let's say,

then after 12 of them, you still have 10.9 days left until another year begins.

So #1). you have to figure out something creative to do with 10.9 days,

and #2). the next year won't start with a Full Moon.

Lots of people worldwide still do use the lunar calendar. Actually, before the romans came, the ancients used the lunar calendar. We don't use it anymore because it isn't useful. We prefer months of equal days, which is not the lunar calendar.