Specifically regarding your low amount of defenses last week... we're kinda puzzled why that was the case for you too. Something about there being magnitudes more players competing in the game relative to the beta has made it so the matchmaking is prioritizing some people in a way that we didn't expect.
I don't want to get into too many specifics (in case there's an exploit to be had which can make people less likely to be attacked), but I have dived into it with a programmer and we're working on adjusting matchmaking values, changing some steps of the matchmaking logic, and ensuring we can sanity check matchmaking requests as though we were players on the live server (without actually affecting what those players would see).
In general, the game tries to populate your attack list with players around your VP and with players who have characters around the same strength as your characters, but it looks like you'll be more likely to be a possible attack list target for someone else at certain ranges of points in the leaderboard.