It's not because of "tribalism".
It has to do with the nature of the membership of each party.
We tend to categorize Liberals and Dippers as "leftists" as if that makes them members of the same species. They're not. They're very different creatures.
Liberals generally respect our free market system. They are very bourgeois and conformist in their thinking, making them realists (or they think of themselves that way). You don't hear a lot of radical ideas from Liberals. They adopt NDP policies after they've been adopted by the mainstream. They're progressive when they think it's feasible and politically advantageous.
Dippers only tolerate our free market system or want it abolished entirely. Their mindset tends to be post-modernist and utopian. They will adopt policies no matter what the electorate thinks of them.
For both, state involvement is central to their policy, but Liberals are "conservative" in their approach to interventionism, only doing what they think is necessary, while Dippers want the State to radically intrude in business and people's every day lives.
And this is why they don't like each other.
They are not going to unite.
It would be like trying to unite a bunch of pot-smoking, anything-goes libertarians with a bunch of buttoned-up, fire-and--brimestone Christian conservatives. They may both hate state interventionism, but their approach to policy is radically different.
I'm surprised more people don't bring this up. It's perfectly evident. Talk of uniting the left is futile.