I may be a tad cynical, but the reason for the reluctance is because they themselves don't want to be the target.. it isn't about PVP but Dominating.. there are pvpers in the game, but many of the most vocal are not true pvpers as they don't want the vs part.. they don't play to have a fair fight, they play to overwhelm players.. they wax eloquently about "risks" and how hard Open is and why it needs x,y, and z over Solo and group, yet they attack vastly weaker opponents and many times in groups vs a single victim... for them there is no risk, no real danger of them losing anything. Put them in a private group with only PVPers and suddenly they have risk... they are no longer an Apex predator.. and that they cannot stand.
It was more or less the conclusion the UO devs reached when Trammel was introduced. Suddenly the "wolves" were playing only among themselves, while most of the "sheep" were safely stowed away, so most of those "hardcore" PvPers started to leave in droves due to the lack of soft targets.
Not all PvPers left, mind; I've seen a few that consider the game just after Trammel was introduced as the best for PvP, as in the PvP world of Felucca you would find only players that were itching for some PvP action. For those, all fights suddenly became life and death affairs, unpredictable, far more exciting than hunting "sheep". Also, not all "sheep" sought protection in the PvE world of Trammel; a few preferred the excitement of being always in danger of player attack. But most early UO PvPers wanted instead to kill easy targets, and most early UO PvE players didn't like being the targets, so as soon as PvE players became able to opt out of PvP they majorly did so, causing the PvPers to run out of soft targets and mostly leave the game.