There are two reasons, at least, that Jeroboam did what he did. One is because he had no choice. It could have been that the Levites may have stood up to him and refused to be priests teaching and participating in error. So they just up and left. If Jeroboam wanted to have a religion, he had to have priests. He could not use the Levites because they were not there. They had refused, so he decided he would put anybody in who wanted to be a priest.
The second possibility is that he may have summarily kicked them out because they represented too much of a threat. What he did, though, or what occurred, ensured that there would be no one who knew God's way well enough to be a threat.
