Reva communicated with his chorus using Telepathy. How come he could not use the same powers when communicating with counsellor Troi?
This is dealt with in the episode. There's no evidence that Riva is a powerful telepath, or even a telepath at all. His Chorus evidently took quite some time to become able to turn his thoughts into speech and it's possible (bordering on likely) that they're the ones with the telepathic gift.
DATA: When Data speaks for me [Riva], can you hear my anguish, my despair? Data is a fine machine, but he cannot take the place of my chorus. It took years to develop a communication. That cannot be easily replaced.
As to why Troi can't simply read his mind, the only time she's been able to do that is with Riker (with whom she is deeply intimate) and various full telepaths such as her mother. Under most circumstances she's empathic (can sense emotions) not telepathic.
Interestingly, in the original script, Riva claims that his Chorus are able to communicate by reading his sign language:
RIVA: My Chorus is so attuned -- they can "hear" my hands and translate to you. It is a relationship which has developed over centuries -- their ancestors provided the same service to my ancestors.
However, it then transpires that he was actually using a mental telepathy device. When that gets destroyed, his Chorus become useless:
Picard, and his staff are there with Riva. Picard has in his hands the "device" which Riva has been wearing.
PICARD: So... communication between Riva and his Chorus was aided by this device. Without it...
TROI: Without it, he is mute.