That's the point though: they can't. There is no rational reason for traders to play in open. By switching to open play, haulers are only exposing themselves to more risk, while not getting anything in return in in-game terms (the added thrill from player interaction is obviously not enough, or even a reason not to for some, as demonstrated by the fact most traders are playing solo or private groups and open play is kinda deserted atm). Pirate NPCs in soloplay just aren't enough of a threat to warrant switching to open play to hire any kind of protection.
The game needs to give positive incentives for traders to come and expose themselves to the extra risk of open play. I'd suggest higher profit margin, to largely make up for the need to hire an escort or the lower profit they'll get from a ship that isn't geared 100% towards cargo space. That way carebears can still enjoy solo or private trading as they are already doing while greed makes the other less risk averse haulers provide content for pirates PVPers to exist, and extra motivation for PVP bounty hunters.
Of course this means fixing the stupid log off problem.
I'm really not sure why this isn't the case already really, considering Frontier made an open play group, but at the same time forgot to give any reason for a sizeable part of the playerbase to partake in it.