I often wonder the same thing. My conclusion has been that automobile infrastructure swallowed the budget.
But I think if you look at modern light-rail projects there really has been such insane cost-inflation it wouldn't be worth covering the city with trams even with a much bigger budget. Also because such a large fraction of the price is admin etc., it creates a bias towards more expensive infra (heavy rail) because the paperwork overhead is similar either way so you get more bang for your buck.