With a few exceptions, catchers are rarely in the lineup everyday, so with the Giants signing Nick Hundley, Trevor Brown's value went down. With Maldonado, Bandy, and now Hundley, the Giants C position is solidified as much as can be without an elite C.
Calica is a local UCSB talent I watched the last couple of years, and he may well develop in time, but getting Hundley for a low level prospect seemed the way to go.