Why are commercial flights not (publicly) trackable in some locations? Flying BA15 from London to Sydney, I noticed that the flight is not tracked over Turkmenistan and Afghanistan, except for a short ping.

Is this for security reasons? Do planes disable their ADS-B, or is the data somehow restricted? Since position tracking is a safety feature, is this regulated or an airline decision?
The reason is that there is no member of the general public in those areas with an ADS-B receiver sharing live data with the public tracking sites.

Websites like Flight Radar 24 rely largely on volunteers installing ADS-B receivers and sharing their received data. They have a good coverage, but some areas are still not covered.