Why are UK primary schools religious but secondary schools secular?

Like many, my primary school experience was fairly religious. We sang hymns, listened to stories from the bible, and recited the Lord’s Prayer every day.

However, my experience at secondary school was the opposite, with literally zero mention of Christianity outside of RE lessons.

Is there a specific reason for this? It seems odd to lean so heavily into Christianity at primary school and then abruptly switch to secular secondary schools.