Because they are relatively complex and unreliable. The Stirling engine used in this application has been under development for at least 30 years (I have a book about it that I bought about ten years ago). The new design uses magnetic bearings and a whole lot of other high-tech stuff that just didn't previously exist. In the book, the working fluid was helium. I wonder wy they switched to hydrogen???
I guess your comment about new materials and computer controls is the real answer to why stirling engines are seeing more use. Someone decided to try and use the old technology with updated processes and materials.
I wonder why they switched to hydrogen???
My guess would be cost.