My take,
1.The ANC is fed up with Mbeki's pomposity and would not tolerate him threatening the constitution.I guess it would be easier to pull a fast one if he had Mandela's gravita or Putin's popularity and command of the party/ country.
2.The ANC did not so much choose Zuma, they wanted Mbeki out.Many members are of the view that Zuma is just too tainted (arms deal, rape charge etc) to be an effective president and his court troubles are not over*.
According to a Markinor poll in May 2006, sample of 1,000 South African Voters) only 26% of South Africans would support Zuma for the presidency of SA **.A November 2007 poll polled Zuma's supporters at 41% (All Africa.com) ***. He is clearly making a comeback
The race for the presidency has just begun, Who will it be? Will Zuma be able to pull a coup-de-grace despite all his problems? He has already been cleared of the rape charges but is bogged by allegations of corruption in an arms deal that prompted the wrath of Archbishop Desmond Tutu, a very well respected anti apartheid institution and elder statesman.
But the most important thing is, if Zuma is to be elected, he has "promised" not much will change (All Africa)****
Is this a story of "Same wine, another bottle?"
Sources
*Arms deal: The truth no one's telling
Mail And Guardian SA (Online) 18 Dec 07
http://www.mg.co.za/articlePage.aspx?area=/insight/insight__national/&articleid=281102
The Economist 15 Dec - 21 Dec 07 "Mbeki v Zuma: the showdown"
http://www.economist.com/world/africa/displaystory.cfm?story_id=10286490
"Few South Africans See Zuma as President" 19 May 2006
"Angus Reid Global Monitor : Polls & Research"
**
http://www.angus-reid.com/polls/view/11934
***
Allafrica.com 18 Dec 07
"Next President - the People's Opinion"
http://allafrica.com/stories/200712180674.html
****
Allafrica.com 10 Dec 07
"Nothing Will Change If I Am President - JZ"
http://allafrica.com/stories/200712100657.html