In other words: who has incentives for attacking pools?
Do you think it's the government?
There are several groups of people that could have incentives to DDoS mining pools:
Owners of other mining pools. Mining pools make profit from the blocks mined by their miners. So it would make perfect sense for them to attack other pools to encourage users to abandon the pool and perhaps find a new home at theirs.
People mining at other pools, including their owners. When a miner is mining for a pool that has issues, chances are high their shares get lost. This means that chances exist that the pool under attack will no longer be able to find blocks and so the hash power of its users is lost, causing new blocks to be found less frequently. This can be advantageous to people mining at other pools because this will result in the difficulty being lowered, giving them a higher chance of finding blocks. (And again, as more blocks are found by another pool, the pool owner makes more profit.)
Opposers of Bitcoin. This doesn't only need to be governments, but can also include banks and payment processors like PayPal, Visa or MasterCard. Bitcoin is very innovative as a payment option and has the potential to take over a significant part of the online payment industry. Also, Bitcoin abolishes the centralized concept of money, from which banks make profit. So they all have reasons not wanting to see Bitcoin succeed as a successful currency and payment method.
One other reason that Steven hasn't mentioned and which I know to be true (having been told by pool operators who have suffered such attacks) that the DDOS attacks have been ransom demands by script kiddies with botnets.
These are simple extortion attempts, and account for many such DDOS attacks.
I believe the main reason is competition.