why was the SW Park obliterated?

[EFG]

I think they destroyed it since they are adding a new district to SW ( Worgen District ) and they didn't want the city to be to big. Since its already bigger than Orgrimmar

There is no worgen district on SW. Nor will there ever be.

[Xeyu]

BUSH DID IT! he got away with 911, now Stormwind!

seriously enough conspiracy, We'll know in time, but it is most likely deathwing.

[Karot]

Deathwing doesn't like parks. When he was a little whelpling, he got bullied at parks a lot. So he vowed that when he got bigger and stronger, that no park would survive and that HE would become the bully!

[Illindel]

The Park really had no purpose for players. Sure druids and shit but the only druids were nelves. Making an entire location for one race is silly. My guess is that they had no ideas on reinventing the park so they decided to get rid of it. Deathwing being there and all kinda made the job easier. Plus, it really puts the situation into perspective. Your home city isn't safe. Deathwing could easily come in and burn it to the ground. Its a pretty clear sign to all races that no place is safe anymore.

Ermm... next stop Dwarven District seeing as we're targeting areas specifically made for one race?

[Shrubninja]

If you'll watch the cinematic, you can see that Deathwing is clearly about to let one rip, and his hindquarters are pointed away from the main gate. The end result was the single most devastating fart in history, destroying not only the entire park, but the ground beneath it.
And nothing of value was lost.