Both men were considered very savvy politicians. Both had powerful national networks of supporters to inform them of the popular will. Both were supremely confident. Both had the power of the government behind them and used it. Both failed to understand the depth of their unpopularity and paid the price.
Incumbency is no guarantee of good political judgment.
If there are crowds demonstrating in the streets, it is wise to listen to them. If there are not, because demonstrations are not allowed, you have cause to be even more worried. The odds are good that a totalitarian or highly authoritarian state that suppresses dissent is a state with a volatile future. The enemy of the status quo is your enemy. Many businessmen and investors were convinced that the South African political status quo prior to 1990 was essential to maintain the value of their investments.