I mean, why not allow remarriage? What if you accidentally kill your spouse or they're a follower and they die?

Because someone would murder all the spouses, post it on youtube, and they'd get a bad rep.

Who's going to get a bad rep?

Bethesda for making a murder simulator. If you can accidentally kill your spouse and remarry, you can premeditatedly murder your spouse and remarry.

I'm not 100% sure this is the reason for one marriage per character, but they obviously think about their reputation when they add things like unkillable children.

But you murder people every day in Skyrim.

I don't. I did when I played the DB/TG questlines, of course, but I've only done those on one character to see what they were like. I doubt I'll do either again anytime soon and possibly never.

Causing the death of another person in defense of myself or someone else is not murder, IMO. Neither is causing the death of a wanted criminal when I'm acting as an agent of local law enforcement. Neither is the killing of enemy combatants in the course of fulfilling my duties as a soldier in an army that is currently at war.

So, killing, yes, lots of it. Murder? Not so much.