The explanation for a missed promise should include what you will do about its effects.

Reasons and repair are not the same thing, though they often arrive dressed alike. A careful account of why something did not happen can leave the other person holding exactly the problem they had before, plus the awkwardness of having to accept the account graciously. What they are usually waiting for is the sentence about the consequence: who covers the gap, when the thing arrives, what happens to whoever was left waiting.

When you next explain a miss, keep the reason short and spend the rest on the remedy. Ask what it caused, then offer the specific thing you will do about that.