Title says it all. A Google search for "bank salt lake city po box" shows that many banks/CUs (Discover, Capital One, AMEX FSB, Schwab, etc...) accept deposits at a P.O. box in Salt Lake City.
Why are they all in Salt Lake City?
Many so-called "back office" functions have been transferred to Salt Lake City in recent years. Many people are surprised to learn that investment bank Goldman Sachs' second largest U.S. presence is now in Salt Lake City. So, much of the customer servicing personnel for certain industries are now based in SLC (even JetBlue, which is headquartered in New York, has many back-office functions performed in the Cottonwood Heights office in SLC), which could very well explain why you see addresses for so many financial institutions listed as SLC. It mainly has to do with cost cutting. If you were a company whose headquarters was in say, New York or California (with high labor costs), you might decide to open an office in a much more cost-efficient location (like SLC) and reduce your expenditures. Once one company starts to do something, many of their industry peers start to follow. Utah also has a number of state incentives to lure business to the state (not unlike Texas). Wages are also, per capita, much lower in SLC versus NYC, San Francisco, Chicago, et al.
No, all bank and credit card companies are not located only in Salt Lake City. Wilmington, Delaware is another popular address for remitting bank payments.
The full story can be found here. Here's the gist of the story:
To lure large banks away from New York, Delaware redrafted tax laws thereby allowing banks to charge any interest they wished on credit cards. Several major credit card companies (Bank of America, Chase, Barclays, and ING Direct) and many smaller banks moved to Wilmington, Delaware for these advantageous tax laws.
Since the enactment of liberal interest rate laws and low taxes, Wilmington has become one of the most attractive cities in the US to set up credit card companies.
In order to make hostile takeovers more difficult, in 1988, Delaware laws protecting companies were made even stronger. Anyone attempting to take over a company would have to own 85% of its stock, or with a majority of stock holding would have to wait three years before initiating takeover proceedings.
As an aside, Senator Orrin Hatch of Utah spearheaded the passage of the Dietary Supplement and Health Education Act in 1994 which supposedly defined and regulated the sale of dietary supplements. In fact, it allows manufacturers to make general health claims about their products without going through the Food and Drug Administration to prove safety or efficacy. The result of the passage of DSHEA is that Utah became the center of manufacturing for such products, generating billions of dollars of revenue for Utah and for Hatch as well as these companies have become Hatch’s top donors over the years.
My guess is that Utah has also enacted business friendly legislation for banks and credit cards and that is why many of them are located in Salt Lake City.
Back in the old days, (say 10 or more years ago) many major credit card companies had PO Boxes in major population centers, and the "Remit to:" in your credit card bill would be hopefully fairly close (based on USPS delivery days) to your home, so that your check would be able to be deposited quickly into the bank's hands. IE: If you live in New Jersey, the "Remit to:" would likely be in New York City. If you lived in Cokeville, Wyoming, your "Remit to:" could be Salt Lake City, Utah. But now, with probably most payments being made electronically, fewer PO box destinations are needed.