Pax Britannica refers mainly to the period of peace caused by the military dominance of the UK, so the largest war in european history at that point would count as ending it.

But it was also there because the UK was the most powerful country in the world during that time, unmatched on the seas (we had trhe Two Power standard where our Navy had to be bigger than the next two combined) and with a vast Empire to call upon in time of war.

However at the end of WW1, Britian was nearly bankrupt, owed massive debts to the US, had exhausted a lot of the material wealth of the Empire to fund the war, and in the meantime other countries had gotton more powerful.

America was now on an equal footing with the British military depleted after WW1, which was exaccerbated with our manpower shortage as so many had died.

So we say it ended then because that's when Britain lost the military dominance it had enjoyed for so long, because Britian and the other european powers had decimated themselves, whilst America (and Japan to a lesser extent) had become significant military powers themselves.