Because if you're going to rationally examine the effect of these drugs in relation to murder, you can't simply limit it to white young people, but have to expand into all races. How many 14 year old black shooters are on psychotic medication? Going to bet the incidence is pretty low. ‘Hold my piece, gotta take my pill’ probably doesn't go well with gang politics.
I think the most likely explanation is that psychotic conditions which would normally be apparent to someone treating them are being masked by both the medications as well as the diagnosis, and it is being compounded by a laundry list of ‘side effects’ that may or may not be related to the medications.
In the category of school massacres and such in that age range, nearly all of them appear to be perpetrated by white kids, not black kids.