This pattern has developed in a number of countries, often following some significant political disruption. For example in Japan and South Africa, a single party wins nearly all elections.
In both countries there was a significant disruption (World war 2, the Apartheid struggle) Similarly in Singapore there was the Malay war and independence from Malaysia. Following this disruption, there was a national consensus. In Japan, the Liberal and Democratic parties had such similar policies that they merged (with approval from the USA). The ANC was the liberation party and gained very broad support because of that.
Then the system becomes entrenched. All competent politicians recognise that if they are to have a chance of political power they have to work within the dominant party. This then acts as a positive feedback loop. Effective politicians join the dominant party, which increases that party's dominance. Other parties are starved of both funding and talent and may become more extreme as centrists find they have a better chance of success with the dominant party. But more extreme parties are less likely to be able to challenge the dominant party, and more likely to split or engage in internecine debates.
Other parties are effectively squashed by the dominant one, even if one-party-rule is not an actual goal of the dominant party and even if there is no actual intimidation of voters.