Around the late 1960's and early 1970s Cantonese Films in Hong Kong were in a decline and a lot more Hong Kong Films were shot in Mandarin especially without sync sound. There's a lot of reasons for this.
1.) The Cantonese Films that were shot in sync sound were still shot in Black And White for the most part and becomin' more cheaply made.
It wasn't until the 1973 Shaw Brothers Comedy The House Of 72 Tenants. And the Films that Hui Brothers did for Golden Harvest. Where Hong Kong Filmmakers started to make Cantonese Films again.
Sammo Hung's Directorial Debut the 1977 Kung Fu Film Classic Iron Fisted Monk and Lau Kar Leung's 1976 Challenge Of The Masters were Cantonese Martial Arts Films gained popularity again. Iron Fisted Monk technically is the first Cantonese Martial Arts Films by Golden Harvest.
2.) There was a law bein' passed in Southeast Asian Countries like Malaysia and Singapore about the Mandarin language especially with Films. In other words demand for the Mandarin Films.