If one takes the first half of the 20th century as a time when late (sometimes too late or overripe 
) romanticism was still alive, I think the impression is somewhat skewed. With the Russians it seems obvious that "modernists" and "romantics" (both very rough and clicheed terms) existed in parallel, sometimes (like Prokofieff) in one person. Among the German/Austrian composers it seems mainly that the big names overshadow the others. But Korngold, Schmidt, Schreker, Joseph Marx, Hausegger and others did exist and they would probably be as well known as some of the British composers if there had not been Mahler, Strauss, Schönberg etc. To put it somewhat malignantly, if there is no first tier in a region, it is obvious that the second tier will be more famous than someone else's second tier.
Another point could of be that the German/Austrian composers of the late 19th century had "exhausted" romanticism to such an extent that more of their immediate successors looked for new, sometimes more extravagant ways of composition. Russian music had a much shorter history but it was also rich enough to produce its "own brand" of both modernism and late/postromanticism (similarly probably for the Czech). But British music was really dormant between ca. 1700 and the late 19th century. (The most important "British" composers between Purcell and Elgar were Handel, Haydn and Mendelssohn...)
There was far more "space" for Elgar and the somewhat younger composers like RVW to do their own particular version of later romantic or also modern music than for a German/Austrian born in ca. 1870.
It is interesting to listen across the decades and think about such things. Certainly in the 1960's, when I first heard e.g.
Prokofiev's Second Symphony, Third Symphony, Chout, etc. (i.e. pre-Soviet
Prokofiev), I would have automatically described him as a "Modernist" and an example of someone breaking away from Romanticism.
Now I am doubtful about that seemingly too easy classification. To be sure,
Rachmaninov's works are more recognizable as 19th-century (post- ?) Romantic efforts, and he would not have agreed (and did not agree) with the younger composer's pushing of tonality. Yet it is difficult to find a more emotional and even emotionally hysterical (in the unpleasant sense of insanity) work than
The Fiery Angel along with its hybrid offspring
Symphony #3. (Think of it as
Carl Maria von Weber's "Wolf's Glen" scene from
Der Freischuetz on steroids
). In fact more and more I hear
Mahler in pre-Soviet
Prokofiev, especially given the extremes toward which both composers seemed to gravitate. No, I am not saying that
Prokofiev knew of or studied MAhler, simply that great minds act in parallel m
Shostakovich famously found inspiration in
Mahler, and the entire symphonic oeuvre of
Karl Amadeus Hartmann, who was nearly a generation younger than
Prokofiev, is very "Romantic" in its expressivity, works which often push against boundaries with nearly manic energy.
Hartmann is often seen as the greatest descendant of the central European symphonic tradition.
Brahms the Progressive is the title of a famous essay by
Arnold Schoenberg, who saw himself as at least a partial descendant of
Brahms, as well as
Mahler and
Bruckner. (See
Dika Newlin's famous book
Bruckner, Mahler, Schoenberg) and who brought the art movement of German Expressionism into music. But was not German Expressionism the further development of Romanticism, a variation into unknown keys, so to speak? Are not
Erwartung and even the later
Moses und Aron full of the DNA of earlier "Romantic" operas, despite the "mathematical coldness" of the 12-tone system? And if Romanticism is about death, love, yearning, and inchoate desires to express things inexpressible, does not "composing with 12 notes" open up new possibilities to explore precisely those things?
