I suspect that the Pro 16 rights only had value for Sky if they got the 6 Nations. Given that they don't appear to have got those, then they won't have a need for other lesser rugby union rights.
If BBC One showed Newsnight after The News at Ten and then the regional news, it would have two and a half hours of continuous news. That is not suitable for what should be a generalist multi-genre channel appealing to the widest range of viewers.