First of all, OP, I'm sorry for you loss. As someone who's witnessed it firsthand, lung cancer is a horrible way to go.
But as someone else said above, the people might not have been taught the right words to say and are tongue tied. This culture has a lot of phobia and denial about death. So people aren't TRAINED in the right thing to say.
I also think your emotions are naturally quite a bit raw right now, rightfully so. A piece of you has been ripped away. So the slightest thing will get to you. Remember anger & irritation are sometimes the other side of grief & depression.
You also have to realize, if you are expecting people to be perfect about this situation, you are setting yourself up to be constantly disappointed and irritated. You truly can choose what you focus on with these people. Focus on and being grateful that these people are expressing sympathy at all, instead of ignoring the situation, your grief, or the topic, instead of the wrong word said.
Would you truly rather people not say anything to you about your loss? Not show any concern or express any sensitivity or sympathy because they don't know the right words to comfort you?
The bolded is a perfect example of what I mean. I'm sorry for your loss, too.