There simply aren't enough engineers in Seattle, especially what I will call "startup" engineers. You're competing against Amazon, Google, and Microsoft (Facebook to a lesser degree) and the engineers in Seattle tend to prefer these established companies over startups. A large chunk of the Bay Area engineering community won't work at Amazon (Facebook and Google are another story) and came there to do startups. It's a bit of a self-fulfilling prophecy of course.

I worked at Twitter when we opened our Seattle office and we were trying to hire maybe 50 engineers the first year. This was before the IPO, trying to hire the ideal skillset for the area (backend) and it was still hard. Meanwhile we had more than 50 offers a week in SF.

The employee base is just so much bigger, it's no wonder companies feel like they have to have offices here.