My take is the significant losses by the NDP were due to two factors- outside that some would never vote NDP, as some see it as a anti business, pro labour socialist party.

1)The last vote gave them significant support in Quebec, but it was soft support garnered by an ill Jacl Layton, versus strong linkages to the party. Mulclair was unable to retain this support, as he was a former Liberal Quebec province Environment Minister. Wiith a less likable style than Layton, he was less able to connect with Quebec citizens. To a degree, pandering to the significant number of Quebec voters on Quebec issues, Mulcair sacrificed some support elsewhere in Canada.

2) Trudeau was the underdog, but faced relentless and personal Harper attack ads long before and dyring tge election. Some folks felt this unfair, and it likely helped Trudeau. Instead of firmly capturing the anti-Harper torch, Mulcair focused much of his energy on attacking Trudeau (as he feared Trudeau could capture anti Harper vites from the NDP). This strategy seemed to backfire, as some saw it as unfair. It enabled Trudeau to be seen as leading the anti Harper charge.