I can't recall?

Demons being the cowards that they are would choose a vulnerable target. Furthermore, in a scene deleted from the theatrical release, but re-inserted into the Producer's Cut, Father Merrin and Father Karras have a brief discussion during a break in the exorcism of why the devil/demon would choose, of all people, to possess a 12-year-old girl. The priests come to the conclusion that, rather than physical destruction being the devil/demon's motive, it is in fact attempting to wreak spiritual havoc by causing people to doubt God.

Father Merrin finds an idol during the excavation that represents an ancient demon. It is clear from his look of recognition that he has seen this image before. Later in the movie, it is confirmed that, years before, Merrin had indeed encountered this demon (Pazuzu, though the demon is never named in the movie) and exorcized it from a young boy (as detailed in the sequel and in the two prequels). Merrin also finds a Christian medallion together with the idol. According to director William Friedkin's commentary on the DVD/Blu-ray editions of the movie, Merrin realizes that the presence of these two objects cannot be coincidence, and sees this as a direct omen, an 'announcement' that the demon will again manifest itself and that he will need to face it again. With that conviction, Merrin says goodbye to his colleagues and sets off to the United States, knowing that his presence and experience will soon be needed there. In order to show he is ready to face his enemy, before he leaves for the US, he visits a larger statue of the demon and confronts it, already feeling the demon's evil influence in two nearby dogs fighting each other.

I think the ouija board played a big part in the demon possessing Reagan. You should never mess with a ouija board; those things are evil!!!!