Ulster is a complicated one. Harsh and cold reality is that part of the reason is a legacy of the First Active takeover and Fred the Shreds approach to management and lending. I know people who worked in UB who were appalled by some of the deals the bank got into after the FA takeover and the UK taxpayer, via RBS, pumped €15 billion into UB to keep it afloat. Reality is that Nat West can see no way to make it profitable. One of the ironies of this is that it's App is streets ahead of it's competitors in Ireland.
There is a pattern here, of arrogance from foreign bankers and not being prepared to adjust to the Irish way of doing things, BOSI is another case in point, they were going to transform Irish banking, that never happened
I also think the difficulties in evicting defaulting borrowers and selling their property in effect undermines and devalues the core asset being held as collateral
But the main reason is we are too small, too dependent on property and Foreign investment to make it worth the banks taking the risk.