I think economics are the best candidate to force it. Until IPv4 addresses get actually scarce, in a practical way that directly impacts people (not just a future, we gotta solve this eventually way), people won't feel the heat.

And before now, there were still some unused addresses available. We've just begun the phase where we have to scrounge for loose change in the couch cushions. Previously there was money in the wallet. (Only a little, but that's what you use first.)

My guess is scrounging will initially be easy. Then it will get gradually harder, until eventually it becomes easier to just use IPv6, and then people will switch over.