about byodoin temple

why Byodoin temple's function is changed from place for pray in to place for torism/museum?

I don't know that it has been changed. Most large temples and shrines in Japan have this duel purpose- tourists visit and often pray there. Often there is a small museum behind the shrine or temple.

Do you mean that there used to be a community of monks there who have now gone?

Strange question. Almost any famed places of worship the world over have visitors as well as fulfilling their primary function of worship, ie, Notre Dame, Chartres, National Cathedral in Washington, Westminster Abbey, etc, etc.