No doubt about that, thats a simple overlooking of game design, and the concepts thereof! The old games didnt use such old-time music outside of the title, it only focused on the brooding dark ambient of mark morgan and isolation. There also wasn't anywhere near the same level of "atomic era" influence. It shows up in some places, but not nearly as much as it shows up in Fallout 3...
Yeah, the swing/jazz is very over powering and pervasive in presence at times, but as three dog says, the radio breathes a life into the wasteland and carries a message of hope for it's inhabitants.
Ever since I played fallout 2, I started getting into cyberpunk and steampunk, in order to make sense of temporal divergences and better understand the orthogonal evolution of technology in alternate realities...
The music is important, as it is one of the game elements that we can "identify" with (elements such as swing/jazz, "green screen" computers, brass knuckles, flame throwers) would have all arrived right before the divergence could have occurred. Our own history would be in sync with Fallout's history until that divergence.
Then, Other elements such as the plasma weapons, would be a result of the atomic technology that continued to evolve past the 1980's, rather than become eventually replaced by the microprocessor, which was developed in our time line's history, now diverged from Fallout's time line. (And still remains as the supreme technological base of our current civilization).
And well, then you have some elements of the game, like the minigun, which was actually a vietnam war era technology, that should not be in the diverged timeline. You could saw it's a kind of overlooked game design concept, but on the other hand, i'm sure that some things are just "bound to happen" regardless of what alternate reality you might wake up one day and find yourself in.
But to answer your question, I certainly do think the game should have more flavor in music, possibly in a manner in which the player might choose from in a manner similar to GTA. But then, you would have to make it all "divergent" music, from all kinds of "new styles" that never occurred in our own world. That would be expensive, if even possible.
I don't know if an explanation for the music used was actually provided in the game, but it certainly could have been done. All popular music could have been in a digital format, and subsequently lost from EMP damage, leaving only old vinyl around. Technically, that would not account for "new" vinyl, (and would be rather lousy plot IMO) unless of course, the diverged time line didnt use that in the way our time line utilizes vinyl (hip-hop, or electronic music such as house, jungle, etc)
I personally really like this music. It carries out the whole world war two era/atomic theme very well in addition to paying homage to classic fallout. But it is in the future though, and so there definitely should either be much more variety there, as civilization did continue for over a hundred more years... Or there should simply be a mere grand total of three lousy crackly vinyls found in some jazz singers casket, dug up, and heavily guarded by the radio station by the brotherhood...
You can see these actual divergences in technology and civilization in classic fallout much more often however, The Orwellian architecture looks liek something from equilibrium. fusion and plasma weaponry is more "high tech" and less "retro" (but not always, necessarily, it's all very convoluted and mixed up, which makes it very very philip k dike in flavor at times...) There also were included constant pop culture references to 101 things like pulp fiction, magic the gathering, army of darkness, mcdonalds, terminator 2, etc. That adds a real nice touch in bridging the disparity and alienation between this orthogonal "future world of tomorrow", and our diverged past for the sake of the player. And in fact, it would actually make for an excellent, excellent update for the DLC, in addition to other temporal "bridges" for the divergence not included in the core game. You could have various kinds of "themes", to replace that old 1950's retro world, in the same manner you can purchase themes for your house in Megaton... Maybe they've thought of this idea, it's certainly a lot of work, but what an amazing game it would create, with a very high degree of replayablity, not only as different character classes, different quests, but different "temporal divergences" to explore the heart of the American Empire through? Hmmm...
Who knows... Maybe they'll do it.