Because they don’t want to.
Well, some of them do. On Christmas Day I was taking the sacrament to the hospital and an error in the hospital census led me to the room of a Protestant who said,”I’m not Catholic but I’d love communion.” I was distressed that I could not give her the sacrament, but we held hands and prayed together for a little while and asked Jesus to come into our hearts and live within us.
Which is merely a symptom of the deeper cause.