Possible answer:

The Institutional Revolutionary Party (PRI) dominated Mexican politics for most of the 20th century and still holds considerable influence in the 21st century. There are several reasons why every election in Mexico was won by PRI for decades, although this trend has changed somewhat in recent years. Some of these reasons include:

Historical legacy
- The PRI was founded in 1929 as a coalition of regional and national elites who sought to consolidate power and stability after a period of political fragmentation and violence.
- The PRI claimed to represent the Mexican Revolution and its goals of social justice, economic development, and political unity.
- The PRI developed a strong party machine that controlled most aspects of government, civil society, and the media, and rewarded loyalty and discipline with privileges and patronage.
- The PRI also used repression, cooptation, and manipulation to suppress opposition and maintain its monopoly on power.
- The PRI thus became deeply entrenched in Mexican society, economy, and culture, and created a sense of identity and belonging among its supporters, who often saw the party as the only viable option for progress and stability.

Election practices
- The PRI established a system of electoral fraud and coercion that allowed it to rig elections and intimidate voters.
- The PRI also cultivated a network of clientelism and vote buying that traded favors and goods for support.
- The PRI controlled the media and used propaganda and censorship to shape public opinion and discredit opponents.
- The PRI manipulated the electoral rules and institutions to favor its candidates and limit the participation of other parties.
- The PRI thus made the electoral process a façade of democracy, while maintaining its authoritarian grip on power.

Opposition weaknesses
- The PRI faced few credible challengers for most of its rule, as other parties were often weak, fragmented, or coopted by the PRI.
- The PRI used repression and cooptation to sideline or co-opt potential rivals, such as labor unions, peasant organizations, and student movements.
- The PRI also benefited from the lack of alternatives and the low voter turnout, which allowed it to win with a minority of votes.
- The PRI thus faced little pressure to reform or democratize, and could rely on its historical legacy and its loyal base to stay in power.

Recent changes
- The PRI's dominance has declined in recent years, as more diverse and competitive political forces have emerged in Mexico.
- The PRI lost the presidency in 2000 to the National Action Party (PAN), and again in 2018 to the National Regeneration Movement (Morena).
- The PRI has also faced corruption scandals, internal divisions, and declining support among younger and urban voters.
- The PRI thus faces a challenge to reinvent itself and regain relevance in a more open and pluralistic political landscape.

In conclusion, every election in Mexico was won by PRI for many years due to a combination of historical legacy, election practices, and opposition weaknesses. However, this trend has changed in recent years, as Mexico has become more democratic and diverse, and the PRI has lost some of its traditional advantages.

Every election in Mexico was won by the dominant party called PRI (Institutional Revolutionary Party).

In its presence, opposition parties did contest elections, but never managed to win.The PRI was known to use many dirty tricks to win elections.All those who were employed in government offices had to attend its party meetings.Teachers of government schools used to force parents to vote for the PRI.Media largely ignored the activities of opposition political parties except to criticise them.Sometimes the polling booths were shifted from one place to another in the last minute, which made it difficult for people to cast their votes.The PRI spent a large sum of money in the campaign for its candidates.