Cause 🆚 Reason

… is a major cause of…

In general, a CAUSE is something that makes something else happen. A cause typically has an inevitable effect. For example:

• CO2 emissions from cars are a major cause of global warming.
• COVID-19 was a major cause of death in 2020.
• Smoking is a major cause of lung cancer.

As you can see in the examples above, the structure [a major cause of] refers to a NOUN (global warming, death, lung cancer, etc.). You CANNOT use this structure with a verb or sentence clause. For example:

❌ Going to school is a major cause of learning a lot.
❌ Her beauty is a major cause of everyone likes her.

These sentences are wrong because they do not use nouns.

…is the main reason why…

In general, a REASON is a motivation or explanation for an action or occurrence. Grammatically, the structure [is the main reason why] can attach to a sentence clause.

For example:

• Her beauty is the main reason why everyone likes her.
• The fact he was under the age of 18 was the main reason why he didn't go to jail for his crime.
• The quarantine policies are the main reason why nobody in my department wants to travel for the upcoming holiday.