Why Commander Riker was never a good character in TNG series ?

I agree. It's hard to imagine that he could have been a better actor. He could pull out a difficult role when necessary, and the creation of Thomas Riker shows that the writers had some confidence that he could do more than support Picard and make a mess of the holodeck. He didn't go the way of Beltran, becoming a resigned player who resented the spotlight moving on, and consequentially, his contributions to Trek as a whole are probably more substantial than Stewart's in the end.