Some branch instructions (B and BL) use an immediate to determine the destination of the branch. Why does the BL instruction require a 4 byte machine code rather than 2 bytes as used by the great majority of ARM Cortex M0 instructions?

A simple branch instruction, B, uses either an 8 bit immediate if it is conditionally executed or an 11 bit immediate if it is unconditional. Why is there a difference between a conditional and an unconditional branch?