In a word, industry. Catalonia is where the industrial bourgeoisie and textile industry took root and with this came an influx of workers from outside of the nation. These workers were often familiar with rural anarchism.

Carlism relies on the notion of "fueros" whereas anarchy syndicalism sees rights as won by working class struggle, Without being unduly economically determinist, it's a question of a change in ideology as the nation came to industrial modernity. The bourgeoise adopted the lliga and then the various republican parties which became the ERC, the working classes supported the libertarian left.

As you note, anarchy syndicalism and communism were deeply distinct entities. Communism never really held sway in Catalonia. Chris Ealham's book "class culture and conflict" is excellent on working class culture in early 20th century Barcelona.