Actually, come to think of it, this problem isn't new. The Wilson governments were noted for ignoring their manifestos.

In the US, party platforms, as we call manifestos, though - in the past - often wrestled over fiercely during the conventions that write them, have absolutely no weight at all and are completely ignored even during the rest of the campaign, let alone in government. Their function during the wrestling process seems to be purely to demonstrate factional strength. Since the parties have become more internally coherent ideologically over the past few decades, even that's faded away, though you'd think platofrms would become more important.