Why do you think monroe opposed the admittance of missouri to the union

Ultimately, Monroe opposed allowing Missouri being allowed statehood because Missouri, back then in the early 19th century), was opposed to slavery.  That's not to say that Monroe was incredibly enlightened about slavery, even for his time.  Still, he was opposed to legal slavery on ethical and moral grounds.  He did not like the idea of admitting a slave owning state to the United States and resisted such.