Why don't we permit a minimum degree of t=1 for a B-tree?
According to the definition of B-Tree, a B-Tree of order n means that every node in the tree has a maximum of n-1 keys and n subtrees. Therefore a B-Tree of order 1 will have maximum 0 keys, i.e. no node can have any keys so such a tree is not possible and as we can't permit a minimum degree of t=1 for a B-Tree.