The obvious answer here is that Bruce never wanted Terry to be Batman in the first place. When the story starts, Bruce has a hard time trusting Terry, and it takes a long time before he sees him as anything other than a hard-headed teen. After so long mistrusting, it would be hard still for Bruce to explain to Terry his emotions at any point (something Bruce was never known for in the first place). The point with Bruce realizing his mortality, and worse, his desperation in that situation, wouldn't be something he would give away freely to people he cared deeply for. It isn't a far stretch to think he wouldn't want to share it with a stranger, even if he was donning the bat suit.
As for the not so obvious answer... throughout the series, Bruce gives Terry guidance and advice, but he leaves him to become his own Batman most of the time. There are several times we see Terry go for a gun and struggle internally about other decisions he makes. If Bruce told him everything, he wouldn't have a chance to grow into his own man and make his own decisions. Bruce knows the Bat was a means for revenge against his parents' killer and eventually turned into the legend known when Terry takes over. When Terry puts on the suit, Bruce wants him to figure out why he feels he wants or deserves the power and responsibilities for himself.