The permanent seats are explained through the practical problem of which states could supply power for major United Nations action. Nuclear possession alone cannot explain the original membership, because the nuclear and institutional timelines do not match. Britain and France had worldwide networks and logistics that supported power projection. Those capabilities made the great powers central to any large force the UN expected to assemble. The follow-up concern is that relative power has since changed, leaving disagreement over how veto reform could work without allowing the strongest state to dominate.
Notable Point · 5:22:24-5:24:07
Why the Security Council veto went to five global powers
The permanent seats reflected the powers expected to supply worldwide reach and force in 1945, while the later erosion of that distribution helps explain modern criticism of the veto system.