Because like it or not Trudeau does have the ability to communicate and connect with voters, So the only thing you disagreed with was his policies.
That and that he was leader of the Liberal Party, a right-wing political party with a documented history of doing the opposite in government of what it promised during the election.
So you don't agree that being PM is NOT an entry level position? Because this will be a MAJOR problem if Jagmeet wins the leadership. Because how can the party go from saying "PM is NOT an entry level position" to then say "Having no federal experience is not a problem"
I think the premise of your question is flawed. All other things being equal, yes I would generally chose a more experienced candidate over a less experienced one. However, all other things are rarely equal in politics, and I'm not prepared to have candidate's experience levels trump all other considerations.
I'll also note that I'm not supporting Jagmeet Singh at this juncture, because everything I've seen suggests that he's more style over substance.