They just don't make good handsets
This is only half true (and I say this without a shred of nationalism; I too once swore that I'd never buy an LG phone, and I'm currently using a Nexus 4).
LG has always made high quality hardware, but their software has always let them down horribly. The Nexus factor solves that completely, giving you good build quality with the stock Android experience.
Unfortunately I won't be getting a Nexus 5 just yet for the sake of my marriage (I only just bought the new Nexus 7), but I'm looking forward to KitKat immensely.