Canada

Did they tell them to go to the back of the line?

Pretty much the reality is that a new election immediately is not in the NDP's interest. They don't have to be in lock-step for anything other than confidence motions and Liberals can support shop between any of the parties for at least 2 votes.

The Liberals are currently being shored up by the Bloc, so no.

Left-leaning policies, parties and politicians have typically aimed to appeal to demographic niches, rather than broadly stated goals that benefit larger subsets of the voting population. Someone like Jenny Kwan, an NDP MP that's been in her seat for decades now and serves as a 'minority rights' type critic, is basically unable to empathize with / represent "average white middle class" voters interests, and it really shows in any communication you have with her. Her political support is almost explicitly rooted in appealing to minority groups, and saying "You have it so much worse than white people, so govt should help you out!", which gains her enough appeal amongst her various niche sub groups to continue to control the riding (even though her riding has gotten jack shit in terms of fed funding for her entire tenure -- they just keep voting against their own interests really).

Because their base has become so entrenched in demographic politics / appealing to racialized groups, any platform that attempts to speak more broadly / appeal to non-racialized groups, presents a potential threat to their underlying base of supporters. Jagmeet, when doing "meet and greets" with the public, instantly and almost exclusively gravitated towards other sikhs -- because his support in the party was largely based on his appeal to that particular minority group, who voted en masse for him because he's Sikh, moreso than his policies/electability.

Put slightly differently, they don't target "traditional" left leaning economic / political ideologies, because their position in the 'new' left is based on appeal to influential minority groups. You don't need to appeal to "everyone", or "as many people as possible", if you can lock down a big minority group, who'll vote for you just because of your race. So you don't see them appealing to the broader public interest. And while that approach works in some segments (Like Jenny's riding, or at NDP conventions), it generally isn't a winning strategy when replicated across the broader voting public. Jagmeet could win his NDP leadership race, because the people voting there skewed heavily into his niche, but he couldn't win the more 'open' race, because his race-based supporters weren't a significant enough slice of the broader population to carry it. Even more, the racial-based support block actually serves to alienate voters of other races -- you can't have a bunch of Sikh people goin "Finally one of us is gettin in, we're gonna see good changes!" without that reading as "We're voting for our own race because we assume there will be race-based benefits / targeted programs to help us as a result! We're voting for racism in our favour!".

And that fear is somewhat justified, unfortunately. I mean, JWR was our first FN AG. She reformed bail to specifically address FN representation in prisons, and is the person responsible for Canada moving to a rotating door for criminals -- she literally revised bail to make it so that LE had to let everyone out asap before their official day in court, because she felt some demographics were over-represented in prison. She also mandated race-based reviews of cases, which has resulted in things like a FN dude who stabbed a white stranger in an elevator, killing him... getting zero jail time as a result, because he was FN and his victim a white guy (happened in Vancouver in 2020). These are moves that are explicitly "bad" for the general public, and arguably bad for equity; a FN AG put in policies benefiting her race explicitly to the disadvantage of everyone else / 'the public at large'. Canada also had Harjit Sajjan from the Liberals, use Canadian spec ops to save non-Canadian Sikhs during the pull out from Kabul -- a fairly clear case where he racially discriminated in favour of his own race, to which the Liberal gov said "He's not racist, cause you wouldn't call him racist if he wasn't a Sikh himself!". Like no shit, someone of a certain race using govt resources to benefit their own race is what people call racist.... but not in Left-leaning politician speak. In left leaning politics, it's ok for minorities to use govt resources that way.

I dunno. I think left leaning parties / politicians have decades of this sort of stuff to try and work through, if they're seriously wanting to try and appeal to the broader "working class". And the political base of the party is not really interested in moving in that direction. Sorta like how the dems in the states were so hard up for getting a woman on the ticket, that they torpedo'd Bernie and alienated a crapload of working class voters. Same general vibe.

Why is the entire political class terrified of the left in so many liberal democracies?

Because the left tries to take wealth and power from the people with wealth and power and give it to the people without wealth and power. You can probably see how people who already have wealth and power wouldn't like that.

Why do even the "left" parties end up fleeing the left for the centre and right?

It's a mix of leftwing politics losing popularity (this happened in the 70s and 80s all over the world and has since reversed, but see above) and opportunists joining parties that were once full of true believers once they become successful.

Even just a worker party would be nice. Not helping to break up unions and such.

Look who controls Canada, its Blackrock and large banks. Hence why the new housing minister calls residential real estate an investment asset, whose price must stay elevated in order to generate profits for investors.