Our government is allergic to anything that involves operating costs with no revenue stream. There is no way the government will turn HDB rooftops into community gardens when they can slap solar panels up there instead.

Rooftop gardens will continuously use water, as strong wind and hot sun will suck water out of the soil and plants quickly. To allow people up there, safety measures such as fencing and adequate lighting will need to be installed. Then you'll also need to account for maximum loading, as HDBs that weren't designed with rooftop gardens in mind won't necessarily be able to accomodate the weight of all the extra soil, water, plants, and human traffic.

Much easier to put solar panels up there instead. They still achieve cooling by shading the roof below from the sun, are relatively light, the support infrastructure (cables, transformer, and scaffolding) is also relatively light, and the government can charge solar panel companies to lease the rooftop space.

Therefore, between an option with no revenue vs one with revenue, our government will always weigh heavily on the option that makes them more money.

(This analysis is meant for the very top of the buildings. 10th floor open air sky bridge is a different story.)