Many young people in Canada at this time, influenced by the 60's counterculture identified with Trudeau, a young, energetic, nonconformist. They were dazzled by Trudeau's charm and good looks, and a large fan base was established throughout the country.
He would often be stopped in the streets for his autograph or for a quick photograph. with this, one could easily see the difference between Trudeau, and other politicians (and reason for the term: Trudeaumania)
many of his fans were attracted to his socially liberal stances (he legalized homosexuality and created more flexible divorce laws). Trudeau was also admired for his laid-back attitude and his celebrity relationships; in that word's prevailing use at the time, describing a modern, hip and happening person, he was described as a swinger.
Pierre Trudeau's Celebrity-esque social position diferentiated him from other politicians.
The popularity surrounding Trudeau at the time can be somewhat equated to the buzz surrounding Barack Obama before the 2008 election. His popularity among Liberals could be equated to the popularity of the Kennedy's among the Democrats.
Canadian: "it's Trudeaumania! everyone's talking about him! he's so cool!"