There are many theories as to why that is, but a personal pet one is that London is lacking in housing stock across the market and as such it is economically more viable to build at the middle and top end of the market where profits are higher than at the low end. When this is combined with a record low investment in local government social housing in London you get London's extreme housing market e.g. demand is high, but supply has all gravitated to the place where returns are highest
It's loose credit. It's the banks.
Perhaps because the parent poster is wrong. Seattle rents did not decrease this year, or last year.
The rate at which rent is increasing has slowed (previously double-digit percentage increases have dropped to single-digit percentage increases). But actual rents have not fallen.
Seattle rents continue to grow each year, just at a slightly slower pace than before.
http://www.bizjournals.com/seattle/blog/2015/11/bad-timingas...
(This is not an argument against building new housing, I also believe we need more of that. But the issue is far more complicated than simply "supply and demand, economics 101")
Ah! Then someone really should have just told me instead of downvoting!
The population of greater London is about the same as that of NYC (~8.5m people), but the population density is half (5,432/km^2 for London vs 10,756.0/km^2 for NYC).