Issue ownership. In spite of parties' convergence in their policy positions on some issues, a party can keep advantage and "own" an issue if its past record in government has given it credit for policy making on the issue.
Non-debate campaigns. In electoral campaigns, rival parties and candidates tend to choose to emphasize different issues, according to different issue ownerships and their expected relative advantage.
Policy consensus. In the long term, broad policy consensus can be accumulated on an increasing number of issues. But in the short term, mediocre policies and incumbent parties with no good performance in government may survive for lack of a sufficiently popular alternative.