What is interesting to watch is in Spain gap between Podemos and UP is pretty small compared to gap between Five Star Movement and Democratic Party in Italy yet latter formed a coalition pretty quickly despite differences, former cannot after 5 months so go figure.
I don't think it's got to do with differences or gaps as much as the fact that parties in Spain are actually 'proper' parties - they have clearly distinct ideology, longer history, larger membership and actual principles (not that UP aren't just being pig-headed at this point), while Italians are more, er, pragmatic. Also, Spanish politicians aren't really used to coalitions.