Why did both the Left and the BJP support the National Front Government in 1989?

They both wanted to keep the Congress out of power.

During the 1990s, there was a notable emergence of influential parties and movements that voiced the concerns of Dalits and backward castes, including the Other Backward Classes (OBCs). A significant number of these parties also championed robust regional identities. These entities played a pivotal role in the governance of the United Front, which came into authority in 1996. Much like the National Front in 1989, the United Front comprised the Janata Dal and several influential regional parties. However, unlike the previous instance, the BJP abstained from endorsing the government this time. Interestingly, the Congress lent its support to the United Front administration, underscoring the precarious nature of political alignments during this period.

In 1989, both the Left and the BJP rallied behind the National Front Government, as their shared objective was to sideline the Congress party from power. In a contrasting scenario in 1996, the Left persisted in backing the non-Congress government, while Congress chose to align itself with the non-Congress and non-BJP forces. This peculiar alignment occurred because both the Congress and the Left were unified in their desire to prevent the BJP from gaining power.