The aircraft itself has a radio beacon that self-activates on a crash. Having one on the black box would be redundant and very likely ineffective since the black box is buried in the guts of the aircraft (e.g. if it separates enough to have a clear field of view for transmitting, it is probably going to be too damaged to transmit).
The ELT is to locate the debris field. (OK, it is officially to locate the survivors, but they tend to be in the debris field.) If the crash is on land, another radio to locate the black box is unnecessary. If it is in the water, a radio transmitter is useless, but a pinger is very useful (see other debate threads why floating black boxes aren't a good idea, and impractical to boot - the armor is very heavy).
