Renee asked:
Why does science tend to be materialistic in its metaphysical assumptions?
Answer by Shaun Williamson
Science is not philosophy and science has no metaphysical assumptions. Science studies the physical world and tries to devise the most powerful theories possible to explain the physical world. However these theories must be fully supported and tested by the experimental evidence.
Science studies the physical material world, that is all. Science has no philosophy, it makes no metaphysical assumptions. However some scientists may stray over the the line between philosophy and science and they may try to persuade us that science favours a particular set of metaphysical beliefs. They are wrong. Science can never lead us to metaphysical conclusions.
Ask a Philosopher 
Science makes no metaphysical assumptions?
Couldn’t be more wrong. Science makes profound metaphysical assumptions about the world that cannot be tested.