Or maybe they waited because they didn't want to damage a royal's reputation? Imagine that, the palace courtiers not wanting to create a public scandal... That's actually the issue that's being complained about here. They've been shielding Meghan all this time. The palace's failure to protect staff from an abusive royal is what the core of this complaint is about, and what Buckingham Palace is now investigating.
The complaint was lodged 2 years ago- not even Markle's camp denies the allegations were made at the time. They aren't new & if you've followed Meghan's coverage (the serious coverage, not just puff piece coverage) over the years you've seen the behavior allegations many times, in the forms of rumors about her behavior, before this 2 year-old email surfaced.