Yes, they should compromise. They're obviously not big enough to win on their own, so they should compromise like all the others. They refuse to do so, and the logical consequence is that other parties refuse to join them.
If you think that having more popularity makes them the absolute authority of the left and every other party should bow to their ideas, we have nothing to discuss.
They refuse to compromise with parties less liberal than them as well, by the way. They refuse to compromise no matter the agenda.