Elections are like trench warfare between large powerful opponents. Heavily contested and lasting over many months, each side’s move is quickly countered, ending in stalemate and eventually a near 50-50 draw, with “noise” preventing precise prediction. This phenomenon is common to both parliamentary and presidential election systems (excluding, of course, today’s nominally democratic non-democracies, in which the incumbent, wielding inordinate power over all systems, dictates the appropriate result, between 70 to 99.XX percent in favor).
The landslide wins of the past can of course re-occur. They can do so when one side has an overwhelming advantage over the other. This advantage can be ‘real’, as in an external factor so large that it overpowers all objections (wars, pandemics, economic crash) , or it can be artificial, such as extreme information arbitrage (media completely biased to one side). An interesting case is when each side brings its own, differing advantages to the election/battlefield.
Whoever would like to hear more about trench warfare, I suggest downloading “Blueprint for Armageddon” from Dan Carlin’s sublime Hardcore History series, in which he lays out the reasons for the bloody stalemates so often repeated in clashes between armies both with lethal firepower but slow movement.