The Taliban are power-hungry and ruthless, while their external (Pakistan/ISI & China) supporters don't mind it.

In an battle between kindness and ruthlessness, (especially where the former is less committed than the latter,) the ruthless will always win. Put another way, if you had one side giving you aid (food/medicine) in exchange for support, and the other (credibly) threatening to kill you for disloyalty, what would you do? You'd probably feign support for the former, and never betray the latter.