[quote=@Vilageidiotx]
Why is it that varieties of Keynesian economics have dominated since the depression?
[/quote]

Because Keynesians were elected.

[quote=Vilageidiotx]
Why were Keynesians elected?
[/quote]

The fiscal and monetary practices espoused by Keynes allow for a wide range of different policies from across the political spectrum to be conducted, whereas maximizing or minimizing state involvement in the economy presents a more definitive tone and allows for a less diverse policy range. In other words, pretending to be Keynesian allows you to act more dishonestly and stick with the political whims of the day, rather than having a spine.