Israel is protecting against at most a few rockets at a time. Shoot the rocket, and it disintegrates into relatively harmless pieces.

Seoul is under continuous threat of thousands of artillery guns, basically lobbing enough boulders to level the city (which contains 1/4 the population of South Korea) in about a half-hour. No modern technology is capable of stopping that kind of low-tech assault on that scale.

North Korea is akin to a drunk with a double-barreled shotgun and Tourette Syndrome: acts irrational, says disturbing things, can cause a lot of harm to one person, and will be put down fast once he does - but can't be stopped prior because attempting to will cause him to harm someone. The hope is to keep him just content enough, as long as it takes, that he'll give up or self-destruct on his own.

Iron dome shoots down rockets with other rockets. The US has some tech in forward operating bases in Afghanistan to shoot down rockets and mortars with 300 round/minute 50 caliber Gatling guns (https://en.wikipedia.org/wiki/Counter-RAM). Both of these technologies only work against explosive/fueled rounds because you can blow them up.

The artilery that North Korea has pointed at the south could be just slug launchers i.e. imagine a giant sized bullet. Because a slug has not fuel or explosives to detonate you can't really stop it by hitting the round with something else.

Seoul is close enough to the DMZ to be within range of more or less standard artillery pieces. These things either shoot shells (giant bullets), or wickedly hilarious amounts of rockets. Even if they were interceptable (I doubt it), the sheer volume of fire would overwhelm pretty much anything.

This is one of the reasons why South Korea created a special administrative region to house a sister-capital city and relocate several ministries and agencies[1] in the next couple of years.

[1] http://en.wikipedia.org/wiki/Sejong_City

I believe Iron Dome is designed to deal with what would be traditionally termed short range weapons like mortars and light-rockets. These weapons are light weight, can be fabricated in comparatively primitive conditions and can be deployed by irregular forces.

The conventional weapons that North Korea could use against Seoul are from the other end of the spectrum. Nearly all of them are patterned after Soviet versions which were designed to fit in a doctrine that favored the deployment and use of large numbers of heavy tube and rocket artillery. North Korea possesses large numbers of 153mm and 170mm guns and 240mm multiple unguided rocket launchers. Many of these are in fortified positions that are within range of Seoul.

Source: http://www.iiss.org/publications/strategic-dossiers/north-ko...

The "Iron Dome" is an anti-missile system. Artillery shells are a lot harder to shoot down. Last I heard, the US military was working on a laser-based system to heat the shells mid-flight and cause them to detonate prematurely. I don't know if this technology is mature enough for actual use, or if it's likely to be deployed to SK.

Artillery fires projectiles, which are hard, if not impossible to stop mid flight. They are actually consideres so stable that many shooting ranges cross over towns.