With reference to the data, although Singapore is smaller than Hong Kong, it has a high vegetables self-sufficiency rate of 10%. The Singapore government even aims for 30% before 2030.
In contrast, self-sufficiency rate of Hong Kong was 30% in 1985 -- a strong proof that Hong Kong had the capacity to grow its food. But now, the rate drops to 2%. Government does not set a target for it neither.
A main reason is the competition with mainland vegetables & urban development.
90% of HK vegetables come from the mainland. Since 1980s low-price mainland vegetables have been imported to HK. At the same time, HK financial sector emerged. Developers purchased farmlands from farmers for development. This caused many farmland abandonment, and a downfall of local agricultural products. What adds to that is the removal of import quota in 1998. This finally caused a mass inflow of mainland vegetables, and the low-sufficiency rate today.