One reason is that the angel of death can't distinguish between the righteous and non-righteous. (Which begs the question "Then how can he distiguish between first born and non-first born," which I think would be harder, but leave that aside.) But then how do you reconcile that with G-d's statement that He, and He alone (not an angel, not a seraph, not a messenger) would pass through the land on that night and do the smiting personally. The commentaries go on to say, that even though G-d is doing the first-born smiting, that doesn't mean that the work of death is suspended. Death is out making his appointed rounds as well. G-d's commandment is to stay indoors and he will keep the Angel of Death from venturing within those doors and killing even those people would otherwise be appointed to die at that time.
I had a different thought. There's some recurring Biblical themes involving watchers. Lot's wife watched the destruction of Sodom and Gemorah and she was destroyed along with it. There's a commandment to, if you steal eggs from a bird's nest, chase the bird away so it doesn't have to watch you kidnapping its children. When the Israelities set up their tents in the desert, the commentaries say that the openings didn't face each other. There was a modesty. A refraining from watching. (This is why Bilam's blessing of the Jews spoke of how goodly their tents were) I feel like there are probably more examples that I've never picked up that exemplify this level of respect tendered by privacy. These are all very different examples, but the one common thread is the emphasis on non-observing. The punishment for people watching in the case of Sodom and Gemorah and Egypt is death because this is an extreme case of suffering. The reward for inculcating a pattern of behaviour that involves not watching others is meritorious, worthy of celebration of blessing, as in the case with Bilam.
I don't think this is leading us to the general conclusion that one should turn away from his fellow, I think it's more to promote a respect for the privacy of the individual. I'm trying to articulate an idea that I see so clearly in my mind that doesn't seem to come out. I think it hinges on the idea of observing passively, gawking, helpless to get involved, but unable to look away. That's the behaviour being targeted. Not loving support, not friendship, or kinship, or real relationships between people, but a genuinely passive disconnect between what's happening and the watcher who's watching.
Hey! What are you looking at?