Why? When both parties compete for the entire vote, voters tend to want to back the winning horse. Many voters don't like wasting their vote on the minor party. Centrist voters also fear vote splitting will give the conservative party a false majority.
This phenomenon is no more evident than in the 2011 federal election. For decades, the NDP was the minor center-left party (averaging 16%,) the Liberals the major party. During the campaign, Jack Layton made a breakthrough reversing roles: the NDP became the major party (31%), the Liberals the minor party (19%.)
End of major/minor effect
Preferential Voting (ranked ballot) will end this kind of polarizing outcome. Under PV, parties no longer compete for the entire vote, just the #1 spot. Since it makes little difference to a voter which party they rank #1 or #2, the whole major/minor dynamic disappears.
With PV, if Liberals campaign from the left and govern from the right, centrists can vote — #1 NDP, #2 Liberals — without worry of vote splitting or wasting their vote. This will make the NDP a major player that forms governments.
PV just as beneficial as PR
The NDP stands to benefit from PV just as much as PR. If one looks at some PV election projections, a center-left party will end up with more seats than proportional at around 24% of the vote. Without the major/minor distortion, this support is much easier to achieve.
Fixing our existing Westminster system with the ranked ballot is a good first step in electoral reform. If the NDP and Liberals make this part of their election platforms, it can be legislated direct. This leaves the door open for a PV/PR referendum, cutting undemocratic FPP out of the picture. This is the safest bet and most practical approach to voting reform.