There are reasons for this. The Catalan parties had failed to deliver independence, and the government had basically collapsed due to infighting. Meanwhile the Socialists had turned down the temperature at a national level, pardoning independence leaders and agreeing to "dialogue". One of the Catalan parties - the Republican Left - had collaborated in this, while the other - Junts - played hardball to extract a general amnesty for those persecuted for promoting independence. And pro-independence voters it seemed preferred the latter approach.
El Nacional has a rundown on the coalition maths, and it is ugly. The PSC could put together a bare majority with (pro-independence) left-wing parties - something which used to be a common arrangement, but which is likely to be unstable now. Or it could form a decent majority with the pro-independence, right-wing Junts - but that's likely to be even worse. Or, it could form a bare majority with the other Spanish parties, but that means working with the openly fascist Vox. The PSC is unlikely to find any of these options particularly appealing. The question is how many of their potential partners would prefer another election in six months...