It's a difference of both market and control.

Who was paying Michaelangelo et al. for those awesome sculptures and paintings? The secular and religious nobles. Typically, today's super rich don't commission religious works and churches don't have the money to commission that level of art. (Most are either struggling to make the budget, putting the money into "real" church work, or paying off sex abuse victims.)

Handel was out to get paid too. He wrote operas, but the church forbid operas during Lent. Religous works were permitted to be performed, hence Messiah.

Christian artists may be Christians, but they are also people who need money to live. Since they no longer have religious patrons, they have to sell to the masses and anything sold to the masses is going to be of less quality and less challenging than that sold to conessiours.