I'll try and keep this simple. I'm averse to permanently killing PC's because my strong suit as DM are campaigns with long, involved storylines that change as the PC's interact with and 'push against' the world. The PC's history of actions and interaction with the NPC's --ideally-- shape the whole campaign. I'm not talking backstories here, I mean what they've done in play. Not to mention PC connection to the various metaplots I scatter about.
Frequent PC death puts a big 'ole wrench in that. Fresh PC's can't, by definition, have an extensive play history. Its makes them orhpans, of a sort. Disconnected. And backstory, while I'm fond of it, can't hold a candle to real, played-out personal history. It's the difference between masturbation and sex, I suppose. And I also digress.
Also, I prefer to challenge players at the level of their character's plots, plans, and so forth. So I replace character death with the lomming threat of failure. That just works better for me. Its breeds more story, rather than grinding it to halt.
Its not that I won't kill PC's. I surely will. But I like to play to my strengths, and I'm convinced a more lethal game would only expose my flaws, without bringing anything else to the table.