From a storytelling point of view, the "Why didn't this person do X?" questions are sort of a waste of time. They didn't do that because something stopped them, obviously. Does it matter what that something was, really? Dragons don't have access to 17th level clerics because they just don't. That's not the way the world works. Assume, if you want, that she asked the Oracle, and the Oracle said there isn't anyone who would be willing to cast it for a black dragon.
And more to the point, True Resurrection is a terrible, narrative-wrecking spell that should not exist, as it has no real purpose for players who die in battle (as they can almost always be returned via simple Resurrection) and only ever comes in to play to undo plot points. I prefer to simply treat it as "not available" to everyone, and I don't want to waste any panel time explaining why.
(Also, with regards to the Gating a Solar thing: The "fair trade" price a Solar would ask for resurrecting an evil dragon would simply be the mother dragon's own lifeóbecause that would be a Good act of self-sacrifice, and the Solar would approve of that. There's no reason to think the mother dragon would be willing to pay that price...and even if she was, she would insist on punishing V first, since her son was relatively weak.)