Popular classical music is rather an odd beast. People's tastes change from year to year, and yet there always seems to be camps of supporters for each style of music - so some tastes don't necessarily change with the season.
There are those that say radio stations like Classical FM cater to boring or conservative tastes. However, it runs a survey every year to catch a glimpse as to what their listeners think are the best 300 pieces of classical music out today. Music from Mozart and Beethoven continue to rank highly among the most favourite pieces to listen to. That said, these same composers are the bread and butter for professional orchestras as well. While it may be nice to perform something new and different, the crowds still are better for music from the masters. Since it takes money to keep these organisations running, performing concerts that sell tickets is a necessity - and so the master's music continues to get performed.
What is also interested about the above linked article, are the types of 20th century music included on this list. Vaughn Williams is the most popular (2nd year running) with The Lark Ascending and third with Fantasia On A Theme Of Thomas Tallis. Elgar's Cello Concerto and Enigma Variations are in the top ten with Barber's Adagio for Strings, Jenkins' The Armed Man, A Mass for Peace and Holst's The Planets (one of my personal favourites) among the top twenty. Yes, all of these pieces are rather tonal, although it could be argued Vaughn Williams' Fantasia or Holst's The Planets have moments which are outside classical tonal structure.
It is interesting none of Britten's music was included in the top twenty (Peter Grimes came in at number 232). Most notably his War Requiem was absent from the entire list of 300 while Jenkin's similar piece (premiered in 2001) came in at number thirteen. I have not heard The Armed Man yet, so I can't really comment on it's style. Given the other pieces on the list and the notes about the influences coming from the French Renaissance song, L'homme armé, I think it's fairly safe to say it stays pretty close to the classical tonal world too.
I am heartened by the appearance of a fairly recent piece making it in the top twenty, as it means there is room for modern compositions to become part of the standard repertoire of classical music audiences. However, other pieces from living composers also made into the top 100, often these are film scores. Shore's Lord of the Rings, Williams' Schindler's List, Morricone's The Mission, Zimmer's Gladiator (although some suggest he used elements from The Planets for his score, so perhaps a nod to Holst is necessary) and Badelt's Pirates of the Caribbean are all within the top 100 and probably owe part of their popularity to the popularity of their films. But even with that, Williams has written scores for over 60 films so there must be something more to Schindler's List than just the films popularity. Certainly the The Mission is old enough (1986) to no longer support the score for its popularity.
So, what sort of music is it that "makes it" into the standard repertoire of a modern orchestra? What sort of classical music is it that appeals to a broad spectrum of people? I think it is fairly safe to say theme is a prime requisite. From the above listed films, theme plays a major role in the pieces. The other 20th century pieces also point toward theme as a major element of music's popularity. The theme from Holst's Jupiter became I Vow to Thee, My Country, an extremely popular British hymn all because of the beautiful theme. Theme and not necessarily tonality tends to be the common thread, at least among the modern compositions, having said that, the tonality of the piece can not venture too far from "classical traditions" either.
Are we locked into a neo-Romantic era, where if it isn't neo-Romantic it doesn't get performed? No, there are plenty of examples of modern composers who are anything but neo-Romantic who are getting performed. However, it does tend to be the neo-Romantic that are getting main stream attention. Again, perhaps Classic FM isn't the best source for what is new in classical music. I do think, however, it is a good judge of what is popular - what is main stream.
As a composer, it is important to be able to compose, to find a way to make a living. The list of choices are, teach (and continue studying, learning growing), film (which, if you can make it can be extremely lucrative - but making it isn't necessarily easy) or commissions - and commissions come when something you've done is successful. So, except for teaching, it is important for a composer to be aware of what is main stream in music. It might be worthwhile attempting something slightly challenging on occasion, but it also is a necessity (IMHO) for the composer to be able to compose in a main stream style, and yet provide a colour, a flavour that is his/her own.
My most recent foray into the classical music world is my Symphony No 1, Figuratively Speaking. The music was written with these thoughts in mind:
The intent was to write something with appeal to a large segment of society. My first symphony, IMHO, needed to be something which proved I could write in the main stream of public perception of classical music. If I succeed, then options two and/or three become obtainable. Other styles of music interest me, but as I find my "voice" in terms of composition, my hope is my "voice" will be heard. As my "voice" matures, it may end up pushing the boundaries of public appeal. However, when it is all over, it would be nice to have one piece that makes it on this list.