Why don’t we allow a minimum degree of $t = 1$?

Consider a hypothetical non-empty B-tree with $t = 1$. By property 5a, the root node has at least one key. Additionally, every node has at least $t-1$ keys and since B-tree nodes cannot have 0 keys without being considered a leaf, we know each non-leaf node has at least one key. Property 5b says each node may contain at most $2t-1$ key and at most $2t$ children. So our $t = 1$ B-tree has at most $1$ key and $2$ children.

Thus $t=1$ is basically describing a form of binary search tree and includes no specifications that cannot also be reached by setting $t=2$. Because of this, and the fact that $t=1$ inaccuratly includes minimum caes that cannot exist, we need not allow $t$ values less than 2.