I can't access anything other than the abstract so apologies if this is in the paper, but personally I think there's a few obvious factors. One is Christian social teaching as a foundation for a lot of the national corporatist models (
Christsoziale Demokratie here in Germany for example), which has always underpinned both the major left and right wings of European politics.
Another factor is a somewhat greater level of homogeneity (i.e 'no Denmark without the Danes') although this seems overplayed usually for political reasons from the more extreme factions given that Europe's social model also seems to hold in recent times despite changing demographics.
Another factor, a little bit contradictory to the last one maybe, is I think also the diversity between individual European nations. There's lots of natural and linguistic borders that have prevented the sort of concentration you see in say, the US or China both politically as well as economically. A lot of the drivers of inequality in the US are incredibly concentrated corporate giants, which domestically in the EU wouldn't have much success.
A fourth factor I think is also the focus on white collar academic work in the US as well as Britain. In particular here in the German-speaking/adjacent sphere (Germany, Swiss, Northern Italy, also Benelux and parts of the East) there's a focus on grey-collar work and industry which provides what Daron Acemoglu labeled 'good jobs'. Middle class vocational professions that support a broad base of workers. The European South has traditionally lacked this and is markedly more unequal.