They are just more rare I believe. The 109 was also longer in production. 1974-1975 was the last of the 84s. But in good condition the 109 can be a valuable item as well. After 1976 they became the black beauty with the plastic baseplate.
Or people are simply more into the Slim as predecessor to the 84.
Guido
Ah, I suspected as much.
Grazie, my friend!