> what is so special about BitTorrent that means it hasn’t been replaced by a hierarchical system?

Every time a top of hierarchy node pops up too high it gets whacked by copyright enforcement. This is an effective pressure against centralisation. Otherwise every decentralised system tends to converge into central systems, like planets forming out of an accretion disk.

Surprisingly the copyright industry is far more effective at driving global law enforcement to do its bidding than the financial regulators are.