On that US map, the state of Utah is shown as having some nice orange up in Box Elder county (top-left of the state). Beyond that, the lesser yellow colors map to the Ogden-Salt Lake City-Provo sprawl.
But the strong Box Elder presence is interesting because it's fairly rural; I found the following article
from 2010, the same year as the map from the OP (Honeyville is in Box Elder, a branch of Jodo Shinshu):
Now, with only 50 members, mid-week services are held just 10 times a year, with no Sunday service offered. The congregation shares the same minister, Rev. Jerry Hirano, with larger and more active congregations in Ogden and Salt Lake City.
But Aoki said her church wasn't always so quiet.
"We used to have more and we used to even have a Sunday school," she said. "But the young people, they get married and not many people come back on the farm. The young people go away to school and they don't come back."
The congregation was a branch of the Intermountain Buddhist Church in Salt Lake City until 1943. It then became a branch of the Ogden Buddhist Church until 1971 when it became independent.
Mike Monson, a minister's assistant at the Ogden Buddhist Church, said the Honeyville Buddhist Church will one day die as its members get too old to attend.
He said at that time, the building will revert back to the larger church and likely will be sold.
It's interesting to see how traditional religious adherence changes after the expatriate generations. For the most part, Mahayana seems to be better suited for export as compared to Theravada, but they all seem to suffer, to one extent or another, from a constricted adherence to cultural expressions.
Related reading:Old wisdom in the New World: Americanization in two immigrant Theravada Buddhist temples