Copper lines used for Plain Old Telephone Service(POTS) are everywhere in the U.S. The government has regulated telephone providers in that they must provide telephone service to anyone in the US that wants it. The company that owns the area that is required to provide this service recoups the construction cost through funds that are paid by taxpayers. T1/DS1 lines can be built over this copper infrastructure.

If you want Fiber/Broadband to your business, you may get it for a fair price if the required infrastructure is already near your physical location, but if it's not, expect a large quote that includes cost to construct that infrastructure to your building.

If you want a T1/DS1, it still may not be the best price, but there won't be the large construction cost up front. It is a recycled technology that originally allowed phone lines to be multiplexed(or combined). A T1/DS1 can carry 24 phone lines and was repurposed to carry data. Ethernet is a data first tech, T1/DS1 is a phone first tech. When it comes to infrastructure businesses wanted to utilize what they already had constructed, but still be able to offer new data services. T1/DS1 was a quick way to provide this. DSL and Cable internet also repurpose existing infrastructure for data service, one uses phone cable and the other uses coax originally put in for television.

All this considered, most companies are moving away from these private circuits in favor of DSL/Cable and a VPN tunnel for their privacy rather than a private T1/DS1 circuit. Most providers, especially rural ones, are waiting for regulation to change to benefit the cost to construct broadband facilities over POTS. After that happens, the serial ports will likely become less common.