Probably the most common question asked about Stirlings is "If they're so good, why don't we have them already?" There are many answers.
First, a lot of ideas don't take off immediately. It took centuries for most folks to accept that the world was round.
The second answer is found in the history of the technology. Steam and Stirling grew up together early in the industrial revolution. Indeed, Rev. Stirling developed the machine in response to the human suffering of steam boiler explosions. But cast iron was the material of that day, and cast Stirlings didn't fare as well as steam engines. Plus, workers were cheap and liability was nil. The fuel efficiency of the Stirling didn't matter when coal was a few cents per ton. So steam won out. If Bessemer had come along sooner with his steel, we might have enjoyed the Stirling age rather than the age of steam.
Once a technology is established and has a constituency, it's difficult to displace. We see that today in the effort to interest the aviation community in a new powerplant. Spark plugs are available, piston rings are available, exhaust valves are available, crankshaft grinding equipment is available, there are experts in every tiny engine component. Mass production makes most of the parts reasonable in price, and the technology is well understood.
Most of the Stirling R&D of the past 25 years has been directed toward the automotive field. This is the ultimate example of established technology versus the newcomer. There is an advantage in Stirlings, but not enough advantage to outweigh the present investment in doing engines the way they're done. (And Stirlings don't make very good car engines, anyway.)
Where in use
There are Stirlings that touch our daily lives. The Stirling is bidirectional, that is, if temperature difference is applied, rotation is produced. But if rotation is applied, temperature difference is produced. So the Stirling makes a refrigerator. If you go to your local welding supply company and purchase liquefied gas (such as liquefied oxygen or liquefied nitrogen), it was made in a Stirling machine. When you watch the satellite weather pictures, they are courtesy of a tiny Stirling cryocooler, used in the satellite to cool the image sensor to near absolute zero. So we do have them already.