It's not a Scion
Seriously though, different reputation, different set of buyers. I think the people who bought a BRZ invested a little more in the beginning so they had to be more certain of their purchase. Meanwhile used FR-S' started to show up so quickly at the same time people complained about "power" that I think it put some people off being interested. Plus if memory serves me correctly, less were made.
I wouldn't be overly concerned with current value unless you're selling. I don't think we'll see this being a long-term production vehicle so it probably will hit a certain point, say $10k and not really fall below that except for poor examples. At the end of the day it's not an exotic so it was never going to have a "stable value." Don't get me wrong, it's worth cherishing while you own it and keeping it in nice condition but for as long as you can buy a new one and the production numbers are decently high, then used prices are always going to fall.