Because Christianity was shown to be a fraud, a scam and a pyramid scheme?
People moved away and most likely started in a NEW/different church building elsewhere.
Churches have a life cycle to them. And, if they somehow fail to keep the younger folks around to continue after the older folks die off, the building will fall into disuse.
Our own church building started in the 50’s, with a small, white, clapboard house on the corner that was converted. The house to the south of it was purchased for the Preacher's home.
Then, as the folks who started it had children and more people also started to attend; the old frame house was torn down (in ‘63) to make room for the fine Bedford stone structure we have today.
The house to the east of the property became available; and it was purchased for the pastors home, and the old one got a large addition added to it, for use as the Fellowship Hall/added Sunday School rooms (in the 70’s)
And about 11 years ago, we determined that we could not grow the size of our congregation due to the limited parking we had, so many trees were removed from behind the church and a larger lot was installed.
Soon after that, we had to enlarge the size of the sanctuary in order to seat the people!
We've kind of leveled out, attendance wise now, due to older members dying and a lot of the younger ones marrying and moving away; but we do have a lot of new kids being born to the ones who have stayed, and there is a renewed vigor being supplied by the young Adults.
They are stepping up and taking on responsibilities and work that we older folks are becoming less able to do.
And so it goes...