My answer was "Because they can."
North Korea has been acting out like this for a LONG time. South Korea and the West (US) would scold them, tell them not to do it again or else, then give them food, money, or whatever. North Korea would quiet down until the next time they wanted something. Then they would act out again and the cycle would continue.
In a perverse way it IS the fault of South Korea and the US/West that North Korea shelled the island. They have caved in to North Korea's bad behaviour for so long no real reaction is expected. North Korea EXPECTS to act out and not get punished. Yes, they have to listen to the US chatter on about it, and South Korea (but not all South Koreans, some back the NorKs.), but that is about the extent of it.
I have said it before and odds are I will use the analogy again. North Korea is a child acting out. It hasn't been properly disciplined from the start. The parents kept letting it do things with verbal warnings and no real consequences. Now there is no expectation of real punishment so they keep acting out. Which makes it harder to actually start taking a stand because it will be a complete surprise to the child and who knows what they would do then.
What are they getting out their latest antics? Isn't the South Korean government taking a hard line against the North now? Well, the South Korean government is now talking about LIFTING the travel ban affecting South Korean businesses in Gaesong, North Korea. Nothing like sending potential hostages to the kidnappers home eh?
It seems like the same old same old.
After 9+ years of living with shit like this happening in one form or another every year, sometimes several times a year, you just stop taking it seriously. (Not to say people dieing isn't serious, I am talking about the actions and reactions.) I don't expect anything to really come of it. It would be nice to see the South and the US actually take a stand. (God knows how badly the North would freak out once they realized it was really happening.) I actually thought it might happen when George Jr was President, but it didn't. I expect lots of noise and nothing of consequence coming from the South/West.