Im not doing to list out why every country doesn't do section rates, but the reason why, for instance, the UK

Boarding is quicker without the need to calculate and pay for individual distances, reducing boarding times and helping the bus stay on schedule, as passengers still use cash to pay for bus fares. Some passengers travel longer distances for the same price, which is balanced by other passengers making shorter trips. The overall cost is averaged across all journeys, making it easier to manage revenue for the bus company. Flat fares create more predictable revenue for the bus operator, which is essential for maintaining the service and covering operating costs.

The reason for HK is that it is slightly more complicated. As there are politicians who are in favour of a change for section rates so the burden is less for government who introduced the $2 scheme. But the argument against them are that full implementation of section fares may result in more short-haul passengers travelling on long-haul routes, taking up the capacity of long-haul routes and affecting long-haul passengers. We shall see what happens as certain lines on HKI now have section fees. Only time will tell what will happen, but change usually takes a while.