They paid £20m for 68% of the club, their shares are probably now worth about 500% of what they paid for them. And only going to go up so long as the club remains in the PL. There really is very little incentive for them to invest any more of their own money - it adds risk and will not return much more than they get for doing nothing
The problem is that if you stand still in football you go backwards. There's a very real possibility that if other teams like Spurs, West Ham, Liverpool and then add in Swansea and Southampton go past us we will quickly find ourselves fighting relegation every year. I also don't think the TV money is likely to continue to improve much. That market is satuated and once the broadband market share war dies down there's a good chance that will decrease not increase.
Right now without investment their shares are probably as valuable as they're ever going to be. They do have a reason to sell.
However I don't think that's the reason they are holding on. I think they can't find a buyer that wouldn't put the club at risk. Call me naive.