Mainly because the evidence (as in, the shot lodged in the walls, the physical construction of the house and forensic evidence in the house) showed that instead of awaking to find he was being burgled and firing off a shot or two into the darkness downstairs to scare off the burglers, as he told the police, he had booby-trapped the house, obtained a shotgun under somewhat dubious circumstances (his having been taken off him for shooting at a car on the road if I remember correctly), removed all the lights from downstairs and lain in wait for who-knows-how-long (I cannot remember if he actually left windows open and so forth); then when the two involved entered the house, he snuck downstairs and up behind them, unloaded the shotgun into them at close range and as they attempted to flee the building, reloaded and fired again as they were running across the garden.A few years ago in the UK, a Norfolk farmer shot and wounded a burglar he found in his house with a shot gun. The farmer was sent to prison.
Thing is, under UK and Irish law, you are allowed do pretty much anything you think you have to to defend yourself in extremis. In other words, someone comes into your home to harm you, shooting them with something or stabbing or bludgeoning them, if it's done in the heat of the moment when you fear for your life - that's completely above board in the law. You're not required to take some sort of vow of never harming anyone.
What you are not allowed to do is to boobytrap your house (injuring a fireman if he's in your house putting out the fire you started by smoking in bed while waiting for the burglars is never going to be well received), or to pursue a fleeing attacker to "teach them a lesson". Lesson-teaching is strictly reserved for the judiciary over here.
The basic premise under the law (and this is very roughly stated and comes from reading case law and legislation - I'm an engineer not a barrister, so weigh it as you will) is that if you're required to defend yourself by someone else's actions then that someone else bears all the legal liability for what happens next - but there's a line drawn between self-defence and "preemptive-self-defence" that boils down to asking very specifically who threw the first punch.
For instance, in another case in UK law, a burglar surprised the owner of the house and after a brief struggle, the owner rendered the burglar unconscious and tied him up. To this point, all above board and legal. The owner then decided to douse the burglar in petrol and set him alight to teach him a lesson. The owner subsequently was successfully prosecuted because the burglar had been subdued and so the owner was not under any conceivable threat when he set him on fire.
So basicly, Tony Martin's not a clear-cut case for good reason, and UK and Irish law is not deficient in permitting self-defence, it's just that the lines are different from in the US (and elsewhere) - and often the actual situation tends to be misrepresented (Martin's case was a fairly well-known example over here because PR seems to have played a large part in his legal defence).
