The short answer is the two-party system. American parties are very good at absorbing/suffocating emergent populist tendencies. Nonpartisan League*, Socialists and Farmer-Labor had their moments in the sun but eventually got absorbed into Democratic Party. Had Social Credit taken foot in the US, I'm pretty sure it would have suffered the same fate and ended up as Democratic-Social Credit Party of Montana or something like that.
The long non-answer is that social credit didn't
really take off in Canada either. Alberta's William Aberhart was a charismatic preacher** who got votes because he was already a household name and presented an appealing alternative solution to the Great Depression - I don't think people cared
what the alternative was. After a serious attempt to make money printer go brrr that lasted about thirty seconds before the feds put a stop to it, and a slightly longer flirtation with quasi-fascism,*** they turned into an ordinary conservative party. Looking at maps of where their remaining support was after they were ousted by PCs, you'll see that it's many of the same areas that later voted Wildrose.
As for other provinces... I don't think BC Socreds ever pretended to be anything other than a conservative party who used the name Social Credit because it was a well-known brand. This was perhaps because they came to power due to non-socialist voters' dissatisfaction with the perpetual Liberal-Tory grand coalition. Saskatchewan and Manitoba Socreds went nowhere. Quebec créditistes were another kettle of fish entirely. They were a reactionary nationalist-but-not-separatist party with populist economic policies designed to appeal to poor but conservative rural Quebec. They were also very, very right-wing on social issues and represented an electoral backlash against Quiet Revolution after Daniel Johnson turned out to be no less radical than Jean Lesage. Towards the end of their relevance their federal wing tried to become a sort of proto-Bloc and were rumored to be controlled by PQ.****
Oh, and in the 1980s federal Socreds almost got taken over by Holocaust deniers and BC's last Socred premier Bill Vander Zalm is now really into chemtrails. Make of that what you will.
* NPL also expanded to Saskatchewan and ran in a couple provincial elections there to disappointing results.
** According to a wonderful theory I heard, Rachel Notley's dad Grant could win in the north of the province because Bible Bill's transmitters didn't reach that far and hence the population was less conservative. What's the opposite of Ahnungslos?
*** From Hash's wonderful blog:
...the inflammatory Accurate News and Information Act. The Act gave the chairman of the Social Credit Board the power to compel all newspapers in Alberta to print government rebuttals or amplifications to any article dealing with government policies and require them to supply the names and addresses of sources. Non-compliance would result in fines or prohibitions on the publishing of the newspaper or some of its materials. The vast majority of the Albertan press was strongly critical of SoCred, pre- and post-election. At the same time, police raided SoCred offices in Edmonton and confiscated copies of a pamphlet which named nine ‘Bankers’ Toadies’ in the province and called for their ‘extermination’.
**** On the night of that Joe Clark confidence vote, créditistes allegedly called Lévesque's office for instructions on how they should vote. Everyone important was asleep and the staffers didn't know what to tell him, so they abstained.
tl;dr it's the two-party system, but social credit didn't take off anywhere - in Canada at least, it was just a label.
