Maybe it's selection bias on my part, but it seems popular Christmas music in the US is heavily represented by recordings from the mid-20th century. Is there a reason for this, and will Bing/Frank/Dean/Nat forever be the faces on the Mt. Rushmore of holiday music?

The below is an expanded version of my earlier answer linked by /u/drylaw in this thread; I wanted to expand on a bunch of the points to provide a bit more detail and historical context (and to focus a bit more on recordings rather than songs, as in the previous answer).

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Mostly, the answer to your question is related to a) the continuing cultural dominance of the baby boomers, and how they related to music of previous eras and; b) the secularisation of American society in the mid 20th century.

If you think about Christmas songs you're likely to hear ad nauseum this month of December while you're in public spaces, or listening to the radio, they broadly speaking can be divided into three categories:

a) ye olde Christmas carols - 'Silent Night', 'Hark The Herald Angels Sing', etc

b) Tin Pan Alley-style mid-20th century Christmas songs - 'White Christmas', 'Have Yourself A Merry Little Christmas', etc

c) modern (post-rock & roll) pop music-style Christmas songs - 'Last Christmas', 'All I Want For Christmas Is You'.

If you look at the list of the most played holiday songs of all time here, the dominant songs are indeed b) - the Tin Pan Alley tunes. However, this list is put together by ASCAP, a publishing rights organisation in the US which historically resisted rock'n'roll and rhythm'n'blues (and including black songwriters, once upon a time); publishing rights organisations like ASCAP administer the copyrights involved in the playing of music in public places, and make sure that the songwriters (or their estates) get the money due to them when their music is played. The ASCAP list there is likely a little biased towards published through ASCAP rather than the competing organisation BMI, or traditional tunes where ASCAP would not be collecting money. As such, the ASCAP list is likely skewed a bit towards the Tin Pan Alley stuff (which is predominantly on ASCAP).

However, your question isn't quite about the songs but about the recordings - the versions of these songs by Bing Crosby, Frank Sinatra, Dean Martin, Nat 'King' Cole, American male singers who would typically get called 'crooners'. Their music, broadly speaking, is the music of the generation before the baby boomers. It sounds very old-fashioned to modern listeners, but once it was pretty revolutionary and new; the crooner style is in stark contrast with the quite-operatic singing styles of the likes of Enrico Caruso. The crooners were effectively the first generation of singers who exploited the capabilities of electric microphones and speakers (which had effectively been around since the mid-1920s), and their music was notably jazz influenced, with a distinct swing to much of it, and often being punctuated with jazzy instrumental solos.

Bing Crosby came to prominence in the late 1920s, basically popularising the particular style in many ways (along with the now-less-well-known Rudy Vallee). Frank Sinatra came to prominence in the late 1930s, and singers like Crosby and Sinatra, along with other crooners vied for prominence over the course of the 1930s and 1940s as the (largely instrumental) swing of the likes of Glenn Miller increasingly came to feature singers, and then watched as those crooning singers became more popular than the band itself. By the post-World War II period - and remember that this is the period that the baby boomers grew up in, seeing as 'baby boomer' as a term refers to the post-war 'baby boom' that occurred after the end of the war - the 'crooner' was the dominant pop vocal style. So during the childhoods of the baby boomers, when they heard Christmas music, it fairly often would have been crooners singing Christmas tunes.

Getting back to the different categories of Christmas tunes mentioned earlier, one very major difference between the carols compared to Tin Pan Alley tunes or the modern pop tunes is the religious nature of the tunes. The carols are earlier music, from a period when Christmas was more explicitly religious, a celebration of the birth of Jesus. However, the trend in mid-20th century America - the period when the baby boomers grew up - was for a repudiation of public displays of religiosity. This was not because there weren't many strongly religious people, but because of a belief that religion was essentially a private matter rather than a public matter, and that importing it into commercial spaces was tacky, and/or not appropriate. As such, Christmas songs in public places during this period that weren't essentially religious tended to be based on 'the season' rather than 'the reason for the season'.

As such, in the 'crooner' period there was a space for new songs which expressed 'the season', especially after the runaway success of 'White Christmas' as sung most prominently by Bing Crosby (written in 1942 by Irving Berlin, a Tin Pan Alley songwriter who would have been more interested in Hanukkah personally). 'White Christmas' was one of the biggest, most consistently year-after-year successful songs of the 20th century. The sheer success of Bing Crosby's 'White Christmas' set off a trend where basically every crooner who followed in the image of Crosby did a Christmas record or seven (e.g., Sinatra's 1948 album Christmas Songs by Sinatra, which was originally released as a literal album of 78rpm records, i.e., with discs packaged like a photo album).

This, effectively, became the Christmas music of the children born in the post WWII baby boom. The baby boomers grew up with this stuff, and I think it's fair to say that they kind of assumed that this music was timeless, rather than actually a relatively new phenomenon of their youth. Generally, the baby boomers rejected this music at the time once the demographic became large and influential enough to be able to make demands of the record industry; typically, they wanted music that was much more energetic and intense (being young people), and that reflected their concerns and beliefs as a generation. As a result the baby boomers themselves gravitated towards musical styles associated with 'rock' - The Beatles, The Rolling Stones, Carole King, Jimi Hendrix, The Eagles, etc - they associated those styles with authenticity where they thought the crooner stuff was generally overly sentimental and maybe a bit boring (though of course, it's a bit more complicated than that in actuality - there's plenty of sentimental Beatles tunes, and plenty of emotionally complex Tin Pan Alley songs).

But Christmas, broadly speaking, continued to be a time when sentimentality was a bit more appropriate, and when the generations came together. As a result, though there was a significant generation gap between the baby boomers and their parents, which was much commented on in the 1960s, the presence of Christmas tunes done by crooners became something that both generations could agree on, even as the dominance of the crooner style faded across the 1960s and 1970s in pop culture. Thus, in a quite symbolic way, you get Bing Crosby - the crooner archetype - and David Bowie, very much a man of rock & roll, singing 'Little Drummer Boy' together in 1977, on a Bing Crosby Christmas TV special.

Musicians associated with rock & roll certainly had a go at Christmas; there's a Beach Boys Christmas album, for example, and A Christmas Gift For You From Phil Spector has ultimately led to the institution that is Darlene Love singing 'Christmas (Baby Please Come Home)'. The likes of 'Jingle Bell Rock' exist. The Beatles recorded Christmas records for their fan club members.

However, there'd be something ultimately pretty weird about, say, the Rolling Stones doing a version of 'Little Drummer Boy' - the kind of 'cool' that is embodied in rock and roll is very much about unsentimentality, and about a certain masculine aggression and posturing. As such, rock bands were typically cautious with keeping a veil of this kind of cool/authenticity on in public. Apparently the working title for the Rolling Stones album Their Satanic Majesties Request was actually Cosmic Christmas and the band toyed with having artwork on the front that was to include a naked Mick Jagger nailed to a cross. Perhaps unsurprisingly their record company Decca nixed this idea, horrified (remember that this was barely a year after there was a scandal after the Beatles' John Lennon said the Beatles were bigger than Jesus amongst kids). But which is probably indicative of a particular rock'n'roll attitude to Christmas. Though in 1979 Keith Richards did release a solo cover of 'Run Rudolph Run' which functioned as a sort of 'I know it's only a rock and roll Christmas tune from the 1950s but I like it'.

But yes - rock musicians typically have an awkward relationship with Christmas. Thus you get something like John Lennon and Yoko Ono's 'Merry Xmas (War Is Over)', where Lennon very deliberately juxtaposes a much more typically hippie political message ('war is over, if you want it') with more crooner-style sentiment ('a very merry Christmas and a happy new year/ let's hope it's a good one'); in parts of the song both messages are sung simultaneously.

In contrast to rock's awkward Christmas-ness, the sentimentality of the jazz crooner style suited Christmas, as far as the baby boomers were concerned. And because their demographics and cultural influence has meant that the baby boomers have had an outsize influence on American culture - I mean, every American President since 1992 apart from Obama was born in 1946 (and for good measure, Hillary Clinton and Mitt Romney were born in 1947 and Al Gore was born in 1948).