I believe it's the same reason that: suppose p is a contradiction; then "p implies q" is true, regardless of the truth of q. I find the reasoning is a little easier to see/accept when we write "if p then q" as opposed to "p implies q." The reasoning is: since there is no case in which p is true, thus there is no case in which "if p then q" is false, therefore "if p then q" is true no matter what q is.

Does this seem to answer your question? If I've missed something, I might not be the right one to provide a full answer.