Because some people can't afford an old one or rather can't give up creature comforts. Besides, supercars are on the verge of downfall due to the eco-mentalist. (Reminds me of Top Gear, the last episode of Season 13).
Well no, it would have went into production, if Megatech (who owned Lamborghini back then) wouldnt have sold the company to Volkswagen, Ferdinand Piech and his henchmen went to italy and said "nein nein nein, gef?llt mir nicht, macht den nochmal von vorne", or shortly, he just didnt liked it.
So they dropped the perfectly ready for production engineerd Cala into the Bin (Millions of engineering costs throw'n away) and started from new and came up with the Murcielago and the Gallardo, engineerd majorly by Audi. ( )