Couldn't is one of the most over used words according to me. I wondered about its various usages and figured its nothing but an excuse for didn't. If you think about it every usage of couldn't can be replaced by didn't and be justified because couldn't comes into usage not because of infeasibility but lack of will.
Like raising your hands assuming yourself to be superman and then saying I cant fly is as good as saying you don't want to unlike wright brothers who did.
And even if we don't look at it objectively its about the kind of gesture one makes which gives an idea about the intent one might not be in a position to do a lot of things but can still do a lot of other stuff to send the right message.