One reason cited is that even though there are a lot more Catholics today, a lower percent attend Mass.
From the Detroit News:
"But the Rev. Paul Sullins, a sociologist at Catholic University in Washington, is researching the distribution of priests, and he said Monday that the shortage may not be a crisis.
* Demand has dropped. In 1965, when 70 percent of Catholic worshipers attended Mass weekly, there was a priest for every 540 weekly attendees, the group most likely to seek Communion. Today, fewer than 30 percent of Catholics go to Mass weekly; there's a priest for every 448.
* Many African and South American nations have three to six times more Catholics per priest than the United States.
* Permanent deacons (married or celibate single men) and parish administrators (including women) handle many tasks once performed by priests. There are 14,106 permanent deacons in the United States who perform sacraments such as baptisms and weddings.
The answer to empty altars is not in changing ordination rules but in redistributing priests, Sullins suggests. Like doctors and lawyers, priests tend to concentrate in urbanized, wealthier locations. They also are tied to the bishop who ordained them. Any change in assignment depends on a bishop's consent. "