Actually, we don't know if purgatory ends on Judgment Day. It could, arguably, continue for some "time" thereafter. Then again, maybe not, perhaps the Last Judgment ushers in a quick searing purification and purgatory passes away with the current heavens and earth. 
 
But perhaps a more fundamental "answer" to your question is to say that there are just some things we don't know. Good Theology must recognize its limits, being content to accept that there are many things God has not revealed. And even in those things He has revealed, we must humbly admit that the mysteries about God and creation have depths beyond our capacity to fully comprehend.

The acclamation you cite was a loose translation of the first acclamation Mortem tuam annuntiamus Domine.... Which is now rendered more faithfully as, “We proclaim your death, O Lord, and profess your resurrection, until you come again.” 
 
There were some requests to retain the "Christ has died" version since it was so familiar, but those requests were not heeded. 
 
The essential problem with that rendering of the acclamation was that it addressed Christ in the third person, speaking about him as if he were not present. All the memorial acclamations speak directly to Christ in the grammatical second person "You" for he is present on the altar, after the consecration, e.g. “We proclaim YOUR death, O Lord ... Save us savior of the world, for by YOUR death ... etc. Hence the acclamations speak directly to Jesus, not about him. 
 
The old familiar acclamation Christ has died...could not withstand this critique and was dropped. Of itself it is a valid acclamation, and can be used in songs, or in other settings, but it is not suited to the moment just following the consecration when the faithful are invited to speak TO Christ in the grammatical second person "you," for He is present.