Should I use past perfect or past simple?

My problem is not the hardest one. Yet, I’ve got confused about how to solve it.

Question is, which tense is better to be used there, past simple (didn’t arrive) or past perfect (hadn’t arrived)? And, could you also explain why?

Speaking of me — I truly presume both are correct, and it depends on what I want to emphasize. However, my English is not good enough, not even close, to answer it myself so that I would be sure that my thinking is right.

Here are the mentioned phrases:

If I was late for registration, it was because my bus didn’t arrive.

If I was late for registration, it was because my bus hadn’t arrived.

I’m looking forward to thanking you!

P.S. My thoughts are,

1. The first one means I could be late, but I actually wasn’t, so there’s no necessity in using past perfect tense.
2. The second one considers that I could be late, but I don’t know for sure whether I actually was late or not. So, it would be essential to point out that if I was, that was because my bus hadn’t arrived.