How is "majority decision" represented in Bitcoin, and why not one-vote-per-IP-address?

Voting is by computing power — "one-CPU-one-vote" — and the majority decision is the longest (most-work) chain. IP-based voting could be trivially faked by spinning up many addresses.

Anyone can allocate unlimited IP addresses (a Sybil attack), so one-IP-one-vote is meaningless. Proof-of-work instead ties each "vote" to a scarce physical resource — computation and electricity.

The majority's decision is embodied by the longest chain, the one with the greatest cumulative proof-of-work. As long as honest nodes control most CPU power, their chain grows fastest and outpaces any attacker's.

Go deeper:

• Sybil attack — Wikipedia — why identity-based voting collapses when anyone can mint unlimited pseudonyms, motivating one-CPU-one-vote.