ᐅ Solid Absorbers – Why Is This Technology No Longer Being Developed?

In the mid-1990s, a housing development was equipped with concrete solar collectors. So-called energy stars or massive absorber panels were installed on the garage roofs. The people I spoke with are consistently very satisfied with them. I had the opportunity to talk to someone who was involved in the project at the time. The whole thing was sidelined by the then chairman of the Committee for Environment and Transport in the Baden-Württemberg state parliament, Kretschmann, with the words "You and your damn concrete," combined with the low energy prices at the time.

Are there still suppliers of this technology?
Why is this technology no longer being pursued?
There is hardly any information about it on Google.
Maybe someone has more information.

It's essentially a heat pump with a brine loop as the heat source, circulating through a slab collector embedded in concrete. The idea is for sunlight to shine on it (in winter?). Whether this is more aesthetically pleasing, cost-effective, and efficient compared to burying the collector underground or simply using air via a fan as the heat source is unclear... There were probably reasons against it, which is why it never became widespread. Its efficiency is likely similar to that of an air-to-water heat pump but lower than one using the ground. The heat capacity is probably limited since the concrete core is slow to respond and cools down.