Why is it that the “east side” of a city seems to be the yucky part?

[CutiePie2000]

I think in Seattle, as you head in an easterly direction, there is a yucky pocket called "Factoria". But there is also Bellevue which is chi-chi and very nice. I don't know Seattle like a local though.

I'm also noticing that yucky neighbourhoods are quickly becoming gentrified and are getting turned into nice neighbourhoods, b/c people don't want to live too far out in the suburbs and would be closer to the city. Vancouver's Yaletown was yucky warehouses and yucky industrial land and it's all nice and 'purty' now.