PET - and a typo! I meant 1980, of course.
Okay, so you're asking why he stayed for four years after winning in 1980? Well, having won the election, it's kind of the traditional thing to do, to serve out a term roughly around four years.
If you mean he could have resigned earlier to give the Liberals time to recover from his unpopularity, I'm not sure why he didn't take that step, except again, it's kind of just not done, to quit that early into a mandate. Though as far as his political vision went, he probably could have left the job right after the Constitution was repatriated in '82, since there really wasn't much else on his agenda to fulfill.
A somewhat darker interpretation would be that Trudeau really didn't care whether or not he hurt the Liberals' prospects in '84, since it is quite well-known that he did NOT like John Turner. And his son Justin, in speaking French to Quebeckers during the 2015 election campaign, let slip that he thought Canada was best when governed by Quebeckers, specifically mentioning Mulroney among the supposedly great PMs from that province. Though I think that if Trudeau sr. was indifferent to the Liberals' well-being at at that time, it was because he didn't like Turner, not because he wanted a fellow Quebecker to win: PET despised Quebec nationalism, and was deeply hostile to the separatists and quasi-separatists that Mulroney had allied himself with.
Basically, though, I suspect Trudeau wanted the Liberals to win in '84, but tradition and ego probably stopped him from quitting earlier.