Why won’t my boyfriend defend me?

I don’t get why my boyfriend will not stand up for me. Im not the type to do anything wrong and try to have my boyfriend defend me. Sometimes when I am in the right really all the time and there is evidence, he still won’t stand up for me. At one point I got blamed for something he even did and he knew it and instead of owning up to it he just stood there and didn’t say anything.
Why won’t my boyfriend defend me?
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