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Why do baptist churches refuse membership to those who have not been immersed? Is it because they do not regard them as Christians, as members of the universal church? Or is it because they require a particular practice of baptism to qualify for membership of them?

James I think you may have hit it on the head. I have seen multiple Baptists sites regarding Local/universal church and a vast majority of them do argue against the entire idea of the universal church. This would explain the entire rebaptism requirement.

However, please keep in mind that many Baptist churches will not accept baptisms, even if they are done by immersion, from a church that isn't a baptist. My own niece was denied access to the elements during communion even though she had been baptized by immersion as a confessing adult. The reason given was that the church that had done the baptism was a "Bible" church not a "baptist" church.

Peter

If you believe what you like in the gospels, and reject what you don't like, it is not the gospel you believe, but yourself. Augustine of Hippo