If you're able to easily migrate issues etc to a new instance, then you don't need to worry about a particular service providers getting shitty. At which point your main concern is temporary outages.
Perhaps this is more of a concern for some projects (e.g. anything that angers Nintendo's lawyers). But for most, I imagine that the added complexity of distributed p2p hosting would outweigh the upsides.
Not saying it's a bad idea, in fact I like it a lot, but I can see why it's not a high priority for most OSS devs
There's something chilling about an expert being asked if a statement represents their opinion, them saying "no", then the statement appearing regardless but attributed to "some experts"