ELI5 why she renounced her citizenship? Not getting it from that post, a 3 minute wander on her site and the FATCA Wikipedia page.
Americans living in other countries don't want to pay taxes anymore. Apparently the only option is to stop being an American, which she has decided to do.
One of the worst parts about this situation is the difficulty in investing from abroad. Can't buy US ETFs/Mutual funds because they don't report properly abroad and can't buy foreign ETFs/Mutual funds because they don't want to report to US. This is a totally unnecessary financial burden on US Expats.