I guess the reason is better composability and fewer wasted opcode "slots". E.g. arithmetic instructions output their result and status flags, and the DAA instr

I guess the reason is better composability and fewer wasted opcode "slots". E.g. arithmetic instructions output their result and status flags, and the DAA instruction takes that as input. Separate BCD instruction would require to duplicate all instructions that are "compatible" with DAA, and there might not have been enough room in the instruction set for those.