By the way, why don't people ever use the phrase "secular Christian" to identify people who come from a Christian background and still do stuff like Christmas trees and Easter dinner.
Because most forms of Christianity are not also an ethnic identity. Ethic identity is formed both as perception by wider society by self-identification. Also the self-identification of of progressive Jews as secular and of religious Jews as conservative has very old roots. For example, if you ever get a chance, see an old Yiddish silent called "The Cholera Bride". If you want to add richness to your undersanding of Sholem Aleichem, think of much of his work in the context of "see deeply religious Jews can be liberal, and secular Jews can be great fools". I mean his satire of the revolutionaries of his time was both because they were often absurd, but also because he felt much of the absurdity sprung from their attempts to cut themselves off from religion, from pruning their own roots, to use a phrase I doubt he ever would have.
Incidentally to see that this is not an isolated phenomena, Christians in Iran ARE seen as an ethnic group. Doubt there is such a thing as secular Christian there, simply because openly saying you are a secular anything would be very dangerous, far more dangerous than simply being Christian. Also, I do know some secular Muslims who call themselves such. Same phenomena, members of a religion that is also a persecuted ethnicity, so people who reject the religious aspects, even the belief in God still identify with a wider Muslim community.