Question

Why are most of the art and sculptures of the Middle Ages either of Christ or of religious events?

Answer 1

Art tends to mimic the concerns of society - in the Middle Ages, this concern was over religion and specifically, Christianity. The entire society was focused around finding salvation - for the poorer serfs and peasants, this was a source of hope for good in a hard life. The authority of the ruling monarch was even based on the supposed relationship between himself and God/Christ.
Also, artists didn't just paint for fun - they were paid and commissioned by churches, the upper-class, and such.
In addition, the black plague which killed more than 25% of the population was seen as related to christianity, and with death, people wished for salvation.
Hope that helps.