It's probably just an unfortunate oversight on Dave's part, and one, I think, that is in serious need of correction. You aren't going to be plausibly able to butterfly rock-n'-roll entirely, not with the African-American cultural influences already in place.
Jycee, unfortunately, the truth is, let's just say that if there is a TOTAL absence, and if it was was intentional, it does not add to the realism, but rather, seriously subtracts from it. I mean, I hope this isn't actually true, but if it is.....well.....
What I meant to say originally is the absence of the definition of rock n roll ads to the realism.
The thing is rock n roll, might still exist but it wouldn't necessarily be known as rock n roll. Or even if it exits it might not be defined in the same was as OTL. Genre is a very fluid thing, what we know as rock n roll might fit into a very broad definition of swing in TTL (especially if we define rock n roll as rhythm and blues by white people) or even jazz. So when David says stomp was influenced by swing, it might as well have been influence by rock n roll.
However David didn't really specify about cultural developments until the 70s. So I guess there is no cannon about what happened from the 20s onwards.
We know black culture was probably repressed in the CSA. Furthermore the plight black during the black holocaust probably caused changes in the themes and sounds of black music. I reckon it became quite a bit less jazzy and quite a bit more bluesy by the end. The repression of black inspired tastes in the CSA, also likely led to blues, jazz, r&b, and derivatives not being exported as much around the world. Especially not to Europe. While, in the USA there was probably no Harlem Renaissance, without black immigration into NYC.
So even if the music exists, which it probably does, it is rather possible and realistic that it wouldn't add to large cultural phenomenon as in OTL. At least not under the same definition.
Sad thing, but as I said instead we get Stomp, which for the most part sounds like the big band version of Rock. While, Bossa Nova is clearly something akin to latin pop, possibly also big band style since it seems to have remained popular in TTL.
There was also a reference to a Metal equivalent in 70s update - only defined as classical music with modern instruments. And I guess Fabrika Punk is essentially Grundge.