Because the teaching of Ayn Rand are the complete antithesis of the teachings of Christ. No sane person could believe both are right.
You know what Jesus taught about God and Mammon.
I suspect like most people they accept bits and pieces of the ideology of others and and fit it to however they think the world works.
They accept Rand economic ideology along with some interpretation of the Bible.
Christian, Republican, Atheist, Democrat are just labels people apply to themselves which may to them mean a variety of things. No two Christians are going to think alike. Takes someone with a lot of charisma to round everyone up and get them heading it the same direction.
Easiest to find a common enemy. Demonize that enemy as much as possible then people will set aside their differences to defeat the common enemy.