It could be that it might have been considered normal, or quite unimportant to the vast majority of people at the time (and subsequent generations) that it just wasn’t documented or remembered. Plus Boudica was quite a way from home, maybe it wasn’t such a big deal to the local people.
And as for the bones maybe they’ve been eaten away completely by certain types of soil? I’m no archaeologist!
If I knew two very large armies were heading my way for a massive fight, I think I’d have got out of the way. Maybe there wasn’t anyone around who could have told you where it happened. Maybe they were more concerned with where their next turnip was coming from.