By safe, I mean it can't be used to blow things up -- either by accident or on purpose. This makes it bad for nuclear Proliferation -- Including making bombs for the US military. That's why it never got as much funding as Uranium-based reactors ... they couldn't figure out how to use Thorium reactors to make bombs. Remember that this was in the 60's and 70's and at the height of the cold war // arms race.
As a side point of not being useful for making bombs, it's also harder to have a (semi) critical accident -- i.e. a Chernobyl / 3-mile-island / China-syndrome type accident.
That's a big problem when you're trying to get the civilian population to accept bomb-making, but (and) much more dangerous Uranium - based reactors... Which would you rather have in your back yard? A Thorium reactor that pretty much can't have a meltdown, or a Uranium one that is one (albeit unlikely) step from being a bomb.
The other problem is that it's too cheap.... from a commercial vantage point, Uranium-based fuels are incredibly hard to make properly -- and their exact format varies from reactor to reactor... That means that reactors that aren't sufficiently profitable to make (even with military^w government subsidies) can make (more) profits because the plant manufacturer has an effective monopoly on making fuel pellets. ... kinda like the way that printer manufacturers sell you the printer for cheap, then ding you on ink refils. LFTR plants, on the other hand, just need an occasional addition of Thorium, and a little bit of the salts (to make up for any evaporation).
That means that there's less likely to be a commercial proponent for LFTR Thorium reactors. Why spend billions researching a reactor that will never get Military/Government subsidies ... and then -- once built -- won't be a lucrative source of fuel sales? It's really good for the utility using the reactor (and their customers), but it sucks for the plant manufacturer.
So there you have it.. LFTR is unlikely to be created because it's.
- Too safe (can't be used to make bombs), and
- to cheap (nobody can corner the market on fuel refills
That's why LFTR may never find a good backer -- unless we can find a billionaire willing to fund the development on a lark (and to save mankind from our own greed/hatred).