The most expensive part of building any car is the labor, and the 113 was a more labor intensive car to build than the 107 series SL. It is entirely possible that if Mercedes were to have built more 113s that they could have sold them but the increased production costs would have eaten up all the profit.
Car companies are in business to make money, the cars are just a by-product.
As for the 190SL, I never liked them, it got to the point where I hated to see them roll into the shop. By the mid 70's when I was working on these cars they were clapped out pieces of junk that the owners (usually car traders) were not willing to spend any money on. They were often purchaced for a few hundred dollars, resprayed with yet another coat of Earl Sheib $29.95 paint, and then they expected a $50.00 flat-rate tune up to fix all the running problems that had been accumulating in over 20+ years of back yard tinkering. When I was an even younger lad in the 60's I thought, the Porsche 356, the 190sl, the 64 Corvette and any of the T'Birds to be some of the ugliest cars around.
My attitudes toward these cars may have softened a little over the years.