TCAS II evolved from TCAS I, which only warned the pilots about conflicting traffic (TA). When advancing an existing system you seldomly begin from scratch but rather keep large portions of the old system and add components and methods which enhance the system performance.
In addition to this historical reason, the TA introduces one possibility to solve the conflict situation, when the crew contacts ATC and ATC acts accordingly.
Because of timing issues with radio the last point can become counterproductive when ATC gives a late advice which comes into conflict with TCAS RA.