Because they don't have to win in order to have power: they can extract concessions from one of the stronger candidates in exchange for dropping out and possibly endorsing him. In Ranked Choice, especially with IRV, third party candidates can be more easily ignored if their 1st choice votes will be assimilated.
In your example, under Ranked Choice, the 15% who really wanted far left policies can be safely ignored after the primary. In traditional voting, the majoritarian center left now has to take their concerns seriously if they don't want to lose the whole thing.
> they can extract concessions from one of the stronger candidates in exchange for dropping out
While technically true, I can't think of a single example of this happening outside of the US two-party system. I.e. you'll see democrats stepping down during the primaries but it's not like Ralph Nader offered to pull out if Gore adopted various policies.
> the 15% who really wanted far left policies can be safely ignored after the primary
The idea is that you wouldn't need to have a "primary" - you could have multiple parties running candidates in the ACTUAL election. The whole reason for the primaries is to get the in-fighting out of the way so you can run a single candidate - because under FPTP anything other than running a single candidate is suicide. With Ranked Choice you could have multiple candidates on the left and right - and people could vote for a first and second choice.
E.g. in 2016 both bernie and hillary could have run. as long as people picked both as part of their ranked choice there would be no negative consequences and people would be free to choose whomever they believed in the most without worrying about their perceived "electability".
In your example, under Ranked Choice, the 15% who really wanted far left policies can be safely ignored after the primary. In traditional voting, the majoritarian center left now has to take their concerns seriously if they don't want to lose the whole thing.