Originally Posted by
razMJ
Because they won't be able to book on a single PNR and check their luggage through. For people going on longer trips (2 or 3 weeks) or with less experience at airports, that is a pain, and they might choose to fly BA the whole way for that reason.
A lot of people do that anyway with VS. Virgin calls them "self-connects". I don't know if that's an industry term.
Plus you can book BA/VS on a single PNR for some flights under their interline agreement.