Again, what exactly would you have them say? I haven't observed them actively avoiding the subject when asked and San Bernadino is more recent example. I'm sure the NYC bombings from a couple days ago and the MN mall stabbings get mentioned more by Republicans and Democrats alike because of recency bias. In any event I'm unsure why you seem fixated on it given the OP subject, are you just clinging to the Charleston thing because you can say "Aha, here's a terror attack done by a white guy!" Attacks by white guys still get mentioned also, see Timothy McVeigh, the Columbine school shooters, etc.