Because that would be ridiculous? The distance between Alaska and California is huge; there is tons of fresh water that is closer to California than Alaska.
From what I understand, there are massive pipelines sending water to the LA basin. And the people who live where those pipelines originate are upset over sending water to what is really a desert. So going further afield in the search of water is likely to exacerbate the tensions.
And I'm not sure where'd they go that they haven't already. Las Vegas? (it has even less water) Yosemite? (probably already feeding water to the central valley) Oregon? (move construction of the Keystone XL westward?)
The distance between Los Angeles and the southernmost point of Alaska's panhandle down the side of Canada^ is comparable to the distance between Los Angeles and New Orleans. Terrain aside, it'd make as much sense for Los Angeles to tap the Mississippi.
Really what California needs to do is suck it up and build desalination plants. They are used in many other parts of the world, despite what some Californian pundits claim about the cost of desalination. Leaching off neighbors is not a long-term solution. Californians need to break themselves of that habit (http://en.wikipedia.org/wiki/California_Water_Wars).
^(And I suspect you'd need to go way further north in Alaska than that to find a Californian-sized fresh water supply...)
Oregon is in drought too, actually, though it is not nearly as bad as California. The trouble with Oregon is that there are mountains in the way — the very same mountains that keep all the rain in Oregon, as a matter of fact. So to sell their water to California, they'd have to pump it over the top, which would negate anything you'd save vs desal. Keystone XL only works in the great plains.
