Why is it "played a main role", not "played the main role" in the example statements below?
Before the invention of transistors vacuum tubes played a main role in electronics.
He played a main role in granting rapid independence for Mozambique and Portuguese Guinea. (thetimes.co.uk)
According to the magazine, Rice played a main role in trying to persuade Egypt, Jordan, Saudi Arabia and the United Arab Emirates to offer training and funding to the Fatah fighters. (theguardian.com)
The passive safety system played a main role in the past and the active systems so-called advanced driver assistance systems (ADAS) will become...
Isn't it that main meaning the most important should be preceded with the article?