I don't think they were unable to deploy, but more like not automatically deployed.
Yes, you must select them manually. They will not
deploy automatically upon touchdown in Direct Law. But perhaps that is something the crew forgot as well.
I understand that, but do you know why it has been designed that way? Is there some technical limitation?
My guess is that in Direct Law it is assumed that there is a major fault in the operating logic and automatisation. Therefore said automatisation is brought down to an absolute minimum, including stuff like automatic deployment/retraction of anything (flaps, spoilers).
That is my guess, but if you think about it - makes sense. Hypothetical scenario - your Main Landing Gear sensors or computer has gone haywire and tells the autospoilers to deploy when you are in the air and really don't want them to deploy. Much worse than you having to pull the handle manually and have them deploy (if you have a clue what you are doing in Direct Law, that is).
Anyone with Airbus experience shed some light if it is the same on the bus?
I do not dream about movie stars, they must dream about me for I am real and they are not. - Alexander Popov