If independence is to be achieved then it is necessary that the ideal should not be owned by one political party. Ideally independence should be supported by all political parties. Realistically, however, this is unlikely—for one thing the ANC is unlikely to support independence. What is needed is that a sufficient number of parties representing a majority of voters in the Cape should support independence. This will help ensure that an independent Cape is a multi-party democracy.
A second reason is that a political party needs to have policies on a wide range of issues. A movement does not have to have policies but instead can concentrate on its core objective without compromise. This in turn makes it possible to talk to all parties and their supporters.
Thirdly, it must be borne in mind that voters are so terrified of the ANC getting back into power in the Western Cape that they are not going to take any risk of this happening. Therefore they are not going to split the vote and instead will continue to support the DA in large numbers.