I agree with the others that it's hit or miss when it comes to TR clowns...
My two TR Ocellaris (ORA) are now hosting in my BTA, but it took them a while to get the hang of it.
They started out hosting with a few inanimate objects, then moved on to my Goniopora, then a Pink Tree coral, and when I removed that, they moved into the BTA.
I think it's a combination of learned behavior and instinct... they have this 'need' for a host, but without other clowns to observe, they proceed by trial and error.
Even though there's no predatory 'danger' in my tank, whenever a powerhead kicks on from my wavemaker and surprises them, they both dash for the anemone and then check to see what it was that startled them... they're much more comfortable (and nicer to watch) since they've taken to hosting the BTA.
As for the ones that don't ever feel the need for a host, well... those are the ones that usually get eaten in the wild.

That's about the only explanation I can come up with...
