What happened to slant-needle sewing machines?
I wasn't able to find any definitive answer as to why the company stopped making slant shank machines. However, I was able to come up with a logical answer. 

Answer: I can only assume its due to production costs. Many sewing machines today come out of the same manufacturing plants. It just makes it easier for all the domestic machines to be on the same low shank, Snap-On presser foot system. 

After World War II, the US started accepting mass imports of cheap sewing machines from Japan. This created stiff competition between Japanese producers and American ones like Singer. 

Singers' response was to differentiate their expensive sewing machines from the cheaper Japanese imports with the slant shank system, which was patented in 1951. They also made their product unique by using aluminum to lighten the weight of their machines (this also made production costs cheaper). 

From there, it was a downward spiral because Singer just could not compete with the cheaper prices of Japanese imports. They too started using Japanese manufacturers and this was the beginning of the end for slant shank.