The question is, why didn't they use this marketing years ago when Zuma was up for election?

they may as well advertise "Suzman would have wanted to stop Zuma!"
Don't get me wrong, I remember seeing ANC banners advertising "Ghandi would have voted for the ANC if he was alive today" which I thought was quite arrogant, but there is no doubt that the DA is also being opportunistic. In fact the thing that annoys me about the DA the most is that it makes the most of publicity stunts like marching to Zuma's house, offering to deliver textbooks when the ANC can't, sneaking Mandela onto pamphlets, etc.
how about they focus on the here and now and rather than shortcomings of each other, what they can do better for the country