Because some important works in the past got opposition, the 'progressive narrative' which claims that developments are determined by 'groundbreaking works' creating a line of relevance with all the other works left by the side, is projected into the past to defend contemporary 'transgressions'. But in reality, many 'groundbreaking works' were accepted quickly or even immediately. Also, in this narrative, mixed reception is ignored and the critique highlighted. Most 'groundbreaking works' were accepted fairly quickly, and music which was received positively, cannot be considered 'unimportrant' for that reason, which is the flip side of the narrative.
A couple of examples:
No work of Mozart was ever booed, during his life and ever after. Yet, he is one of the greatest composers of the ‘canon’.
Beethoven's Eroica got a mixed reception, some people found it too difficult and too long, some people immediately recognized it as a masterpiece.
Chopin's 'avantgarde' music was accepted immediately by the elite for whom it was meant (the Parisian salons).
Most of Brahms' greatest works - the requiem, the symphonies, the violin concerto, the 2nd piano concerto - were accepted immediately. Only the 1st piano concerto was severely criticized for its harsh sounds, and only got 'off the ground' in the 20th century, which does not mean that therefore, it is a better piece than the rest.
Wagner's operas went down very well with audiences; it were the theorists and the critics who got wound-up with protests. Tristan bumped into polite incomprehension for a long time, but Meistersinger, Wallküre and Parsifal were an immediate success.
Most of Strauss' works were immediate successes, even Salome and Elektra, which were highly dissonant sensations.
Mahler always got mixed reception, not because his music was 'too modern' for the bourgeois ears, but because of his 'lapses of taste': including 'vulgar material', folky stuff etc.
Debussy's Faun prelude was an immediate success and had to be repeated, while the piece broke with almost all traditional rules. His opera Pelleas et Melisande met strong resistance in the beginning (1902) but within a year it was a sensational success and the production was repeated for many years.
Schoenberg's music was almost always received negatively, not because it was 'too modern' but because audiences felt this music was breaking-down the art form.
Stravinsky's Firebird and Petrushka were great successes immediately, as was the Sacre (the premiere, which ended in scandal, was as a ballet for a traditional ballet audience, the concert performance a year later was an immediate sensation).
And so on and so forth. A cow is an animal, which does not mean that every animal is a cow. The modernist myth is pure fabrication, to sideline critique. It was, and often still is, a way of defending an indefensible position, and a most practical instrument in the hands of the untalented - mostly the only instrument they can handle.